Multi-Stage Construction of the Little Cottonwood Stock, Utah: Origin, Intrusion, Venting,Mineralization, and Mass Movement

Brigham Young University BYU ScholarsArchive

Theses and Dissertations

2019-07-01

Collin G. Jensen Brigham Young University

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BYU ScholarsArchive Citation Jensen, Collin G., "Multi-Stage Construction of the Little Cottonwood Stock, Utah: Origin, Intrusion, Venting,Mineralization, and Mass Movement" (2019). Theses and Dissertations. 7552. https://scholarsarchive.byu.edu/etd/7552

This Thesis is brought to you for free and open access by BYU ScholarsArchive. It has been accepted for inclusion in Theses and Dissertations by an authorized administrator of BYU ScholarsArchive. For more information, please contact [email protected], [email protected]. Multi-Stage Construction of the Little Cottonwood Stock, Utah: Origin, Intrusion, Venting,

Mineralization, and Mass Movement

Collin G. Jensen

A thesis submitted to the faculty of Brigham Young University in partial fulfillment of the requirements for the degree of

Master of Science

Jeffrey D. Keith, Chair Eric H. Christiansen Michael Dorais

Department of Geological Sciences

Brigham Young University

Multi-Stage Construction of the Little Cottonwood Stock, Utah: Origin, Intrusion, Venting, Mineralization, and Mass Movement

Collin G. Jensen Department of Geological Sciences, BYU Master of Science

The Little Cottonwood stock in central Utah, USA, is a composite granitic pluton that hosts the White Pine porphyry Mo-W deposit towards its northeast margin. The deposit is centered on the smaller White Pine intrusion, and associated igneous units include the Red Pine porphyry, phreatomagmatic pebble dikes, and rhyolite dikes. Twelve new U-Pb zircon LA-ICP- MS ages, for samples from this deposit and in pebble dikes from the nearby East Traverse Mountains, give peak ages of about 30 Ma and 27 Ma for the Little Cottonwood stock and White Pine intrusion, respectively, which correlate well with ages from previous studies. Ages of about 26 Ma were obtained for the previously undated Red Pine porphyry.

The ages of the Little Cottonwood stock, White Pine intrusion, and Red Pine porphyry, as well as disparities in whole rock elemental differentiation trends, suggest that these units are magmatically distinct, and are not simply derivatives of one another with varying degrees of differentiation. Quench textures and resorbed quartz in the Red Pine porphyry are evidence that the magma system vented, which probably produced volcanic eruptions and emplacement of pebble dikes nearly synchronously with quartz-sericite-pyrite alteration and Mo-W mineralization. The mineralogy and geochemistry of these units imply that the magmas formed in a subduction-related magmatic arc setting rather than in an extensional basin related to orogenic collapse.

Pebble dikes in the East Traverse Mountains 17 km away contain igneous clasts that resemble the units in the White Pine deposit in texture, mineralogy, and in U-Pb zircon ages. This supports other recent studies that suggest that the East Traverse Mountains rested atop the White Pine deposit prior to being displaced in a mega-landslide, and the pebble dikes in both locations are the top and bottom of the same mineralized phreatomagmatic system.

The construction of the pluton began with intrusion of the Little Cottonwood stock, then the White Pine and Red Pine magmas. Fluid exsolution from the Red Pine magma accompanied venting, inception of the mineralizing hydrothermal system, and quenching to a porphyritic stock. Pebble dikes intruded into the overlying East Traverse Mountain block, which catastrophically failed millions of years later and was emplaced in its current location.

Keywords: Little Cottonwood stock, granites, porphyry Mo deposits, U/Pb geochronology, hydrothermal alteration, pebble dikes, landslides ACKNOWLEDGMENTS

Funding for this study came from the BYU College of Physical and Mathematical

Sciences, in part from the College High-Impact Research Program. Many thanks go to the many

professors and students who helped this project become a thesis. First, my thesis advisor Jeff

Keith, who first came up with the idea that the East Traverse Mountains are a landslide, and who

provided constant encouragement as I worked on the project. I also extend thanks to the others

of my committee: Eric Christiansen, who gave me direction and helped me work through many

of the ideas in this paper, and Mike Dorais, a source of help with much of the petrography and mineralogy. Thanks also go to others who provided valuable thoughts and insight or field expertise on the project: Ron Harris, Bob Biek, Ken Krahulec, and Tim Thompson. Scott Thayne took us on a tour of the Geneva Rock gravel pit on Traverse Mountain to collect samples.

Several people helped with analytical lab work both at BYU and at the University of Utah, including Mike Stearns, Diego Fernandez, Grant Rea, Mike Standing, David Tomlinson, and

Tober Dyorich.

And much thanks for many hours of field work and lab work go to the students of the

Traverse Mountain Army, including Ryan Chadburn, Bret Young, Spencer Burr, Cameron

Harrison, Chris Perfili, Lars Jordan, Thane Kindred, Riley Dunkley, Tia Misuraca, Alec Hoopes,

Sam Martin, Alec Martin, Joseph Tolworthy, and Steven Hood. And, of course, tech support, emotional support, and financial support of my friends and my family allowed me to finish this intriguing study, for which they will always have my gratitude. TABLE OF CONTENTS

List of Figures ...... vi

List of Tables ...... vii

Introduction ...... 1

Geologic History of the Region ...... 3

Geology of the Wasatch Igneous Belt ...... 5

Methods...... 7

Geology ...... 9

Zircon U-Pb Ages ...... 9

Unit Descriptions ...... 11

Little Cottonwood Stock ...... 12

White Pine Intrusion ...... 17

Red Pine Porphyry ...... 23

Pebble Dikes and Breccia Pipe ...... 24

Rhyolite Dikes ...... 29

Geochemistry ...... 31

Discussion ...... 34

Origin and Evolution of the Magmatic Units ...... 34

Tectonic Setting ...... 34

Origin of the White Pine and Red Pine Units ...... 38

Y and the “Adakitic” Signature ...... 39

iv Rhyolite Dikes ...... 41

Construction of the Pluton ...... 42

Evidence for Venting ...... 43

Molybdenum-Tungsten Mineralization ...... 45

Connection to the East Traverse Mountain Landslide ...... 51

Pebble Dike Evidence Supporting the Landslide Hypothesis...... 51

Possible Alteration Contribution to the East Traverse Mountain Landslide ...... 52

Sequence of Events ...... 53

Conclusions ...... 55

References Cited ...... 57

Appendices ...... 65

v LIST OF FIGURES Figure 1. Geologic map of the Little Cottonwood Stock and East Traverse Mountains ...... 2 Figure 2. Geologic index map of the region ...... 3 Figure 3. Geologic map of the White Pine Mo-W deposit ...... 7 Figure 4. Probability plots of zircon ages ...... 11 Figure 5. Chemical variation diagrams of the intrusive igneous units ...... 14 Figure 6. QAP classification diagram of the intrusive igneous units ...... 15 Figure 7. Photomicrographs of the White Pine and Red Pine units ...... 19 Figure 8. Major element variation diagrams of the intrusive igneous units ...... 23 Figure 9. Photograph and photomicrographs of the East Traverse Mountain pebble dikes ...... 28 Figure 10. Spider diagram of intrusive igneous units ...... 31 Figure 11. K variation diagrams showing extent of alteration...... 32 Figure 12. Si-Y and Sr/Y-Y variation diagrams ...... 34 Figure 13. Sr-Nd isotope diagram of units from White Pine and Bingham Canyon ...... 37 Figure 14. Cartoon sequence of events diagram ...... 47

vi LIST OF TABLES Table 1. LA-ICP-MS U-Pb zircon ages ...... 9

LIST OF SUPPLEMENTAL DATA

Table A1. Modal mineral abundances…………………………………………………see attached

Table A2. XRF whole rock data……………………………………………………….see attached

vii INTRODUCTION

Porphyry-style ore deposits are associated with mineralizing hydrothermal systems related to igneous intrusions. In some locales there is evidence of sequential emplacement of multiple intrusions that form a porphyry deposit, such as at the porphyry Cu or Mo deposits at

Climax (Audétat, 2015), Henderson (Carten et al., 1988), and Mt. Emmons (Thomas and Galey,

1982) in Colorado; El Salvador, Chile (Gustafson and Hunt, 1975); Pine Grove, Utah (Keith et al., 1993); Agua Rica, Argentina (Landtwing et al., 2002); Questa, New Mexico (Gaynor et al.,

2019); and Endako, British Columbia (Villeneuve et al., 2001).

The White Pine porphyry Mo-W deposit is hosted in the Oligocene Little Cottonwood stock (Fig. 1). The White Pine intrusive phase in the north-eastern part of the pluton, where the low-grade ore deposit is centered, has been described as a more leucocratic phase of the pluton

(Sharp, 1958). There has been some debate, though, as to whether the White Pine represents the last highly-evolved portion of the Little Cottonwood stock magma that crystallized inwards

(John, 1989; Smyk et al., 2018), or if the White Pine is a separate magma that intruded into the

Little Cottonwood stock (Sharp, 1958; Crittenden, 1965; Marsh and Smith, 1997; Krahulec,

2015). Also uncertain is the relationship of the White Pine intrusion to the Red Pine porphyry, which has been neglected by workers since it was first described by Sharp (1958).

We considered this question as part of our study of the Little Cottonwood stock and

White Pine deposit, and the possibility of a landslide origin for the adjacent East Traverse

Mountains. This required detailed comparisons of the petrology, petrography, and ages of the various intrusive units of the deposit, including intriguing pebble dikes in the deposit and in the

East Traverse Mountains, the composition of which are integral to testing the landslide hypothesis. This is a study of the igneous petrology of the associated intrusive units of the

deposit, as well as the sequence of events from the intrusion of the Little Cottonwood stock, to

the formation of the Mo-W ore deposit in the White Pine phase, to the production of the pebble

dikes central to the landslide question, which may have been accompanied by volcanic activity.

The purpose of this study is to 1) trace the genesis of each of the major felsic units that

comprise the deposit, including how the regional tectonic setting influenced them, 2) examine a

possible connection between the intrusion of the pluton and the proposed East Traverse

Mountain landslide, and 3) outline the sequence of events from intrusion of the stock, venting, mineralization and alteration, and occurrence of the landslide.

Figure 1. Geologic map of the study region, including the Little Cottonwood stock with the White Pine Mo-W deposit, and the East Traverse Mountains. The proposed approximate former and current extents of the landslide are noted in dashed yellow lines. Locations of the pebble dikes in the White Pine phase and East Traverse Mountain are shown by green circles (not to scale). The pseudotachylyte/cataclasite shear zones are shown by thick black lines.

Geologic History of the Region

Most of the basement rocks of northern Utah are thought to be Archean and Proterozoic

crust that was accreted onto the North American Craton at approximately 1.9-1.8 Ga (Whitmeyer and Karlstrom, 2007). Some workers, including Presnell (1998) and Vogel et al. (2001), propose that the east-west trending Uinta-Cortez Axis was formed along the suture zone between the

Archean Wyoming Province to the north and Paleoproterozoic accreted terranes on the south

(Fig. 2). According to Vogel et al. (2001), the Little Cottonwood stock and the middle Cenozoic

Wasatch Igneous Belt of which it is part were emplaced along this structural weakness. Nelson et

al. (2011) dispute this, showing that statistical analysis of ages and isotopic compositions of

granitic and basement rocks of various ages place the Cheyenne Belt suture zone north of Utah.

This casts doubt on the hypothesis of the Wasatch Intrusive Belt marking a basement suture

zone, and places the entire study area in Paleoproterozoic accreted arc terranes.

Figure 2. Geologic index map of the study region, including tectonic features. The Uinta-Cortez axis is shown as striped line. The area of Fig. 3 is shown by a grey box. After Vogel et al. (2001). In the Neoproterozoic, ca. 780-680 Ma, the supercontinent Rodinia broke up, opening up

a new ocean basin and placing Utah and eastern Nevada on the newly created western margin of

Laurentia (Whitmeyer and Karlstrom, 2007; Yonkee et al., 2014). This led to a period of

deposition of a miogeoclinal wedge along the cooling passive margin, pinching out at a hingeline

along the present-day location of the Wasatch Mountains (Dickinson, 2004). Although the Antler

and Sonoma orogenies happened farther west, deposition in the miogeocline wedge continued

through the Paleozoic.

In the mid- to late- Cretaceous, the Sevier thrust belt of the Cordilleran Orogeny caused

crustal shortening, stacking miogeoclinal sediments. In the central Wasatch Mountains, this is

manifested in the Charleston-Nebo (or Provo) Salient, an eastward-curving thrust sheet comprised of allochthonous Pennsylvanian-Permian miogeoclinal marine rocks known as the

Oquirrh Group, which make up much of the Wasatch Mountains in the region (DeCelles, 2004).

By the end of Cordilleran crustal shortening and thickening in the early Cenozoic, a high plateau known as the Great Basin Altiplano had developed (Best et al. 2009).

Later the Mid-Cenozoic Ignimbrite Flareup occurred across the western United States.

Volcanism swept southward over time, perhaps related to roll-back of the subducting Farallon

Plate, in which the previously flat-subducting slab foundered from north to south, allowing hot asthenosphere to come into contact with the subducting lithosphere and generate magma (Best et al., 2009, 2016). It was during this time that the Wasatch Igneous Belt was emplaced, from ca.

36-26 Ma (John, 1989; Vogel et al., 2001; Smyk et al., 2018).

Crustal extension in the Altiplano started in the late Cenozoic, causing rifting in the Basin and Range by around 18 Ma (Best et al., 2009). This was probably driven by back-arc spreading, relaxation of compression due to a change from shallow to deep subduction, and later on the cessation of subduction as the western margin of the continent transitioned to a transform plate boundary (Zoback and Thompson, 1978). This extension of the Altiplano led to the formation of

the Basin and Range province, which is bounded on the east by the Wasatch Front range, a result

of the west-dipping Wasatch normal fault.

Geology of the Wasatch Igneous Belt

The Little Cottonwood stock is part of the Wasatch Igneous Belt, an east-west trend of

eleven Tertiary intrusions and a volcanic field in Central Utah, aligned along the Laramide-age

Uinta-Cortez Axis (Fig. 2). From east to west, the igneous rocks of the belt generally decrease in

age (from 36 Ma to 26 Ma), increase in silica content, and increase in emplacement depth (John,

1989; Hanson, 1996). The Wasatch Igneous Belt intruded into cratonic rocks overlain by the

allochthonous Charleston-Nebo thrust sheet, comprised mostly of Paleozoic miogeoclinal

sedimentary rocks, especially quartzite and limestone of the Pennsylvanian Oquirrh Group (John,

1989).

The Little Cottonwood stock is the westernmost of the Wasatch Igneous Belt intrusions,

and is the largest (115 km2), youngest (30.5-29.2 Ma), most silicic, and most deeply emplaced.

The western margin of the exposures of the Little Cottonwood stock is the Wasatch Fault, a

west-dipping normal fault that comprises the boundary between the Rocky Mountains and the

Great Basin. Up to 11 km of offset along the Wasatch normal fault since 12 Ma, at a slip rate of

0.6-0.7 mm/y, caused the intrusion to tilt about 20° to the east (John, 1989; Kowallis et al., 1990;

Bruhn et al., 2005). This gives rise to incongruous depths of crystallization across the intrusion:

fluid inclusion data gives a depth of 6 km on the eastern margin near the White Pine phase, and

11 km on the western margin (John, 1989).

The intrusion is compositionally and texturally zoned from a finer-grained less-silicic outer zone to a coarser-grained more-silicic zone in the center (Marsh and Smith, 1997) . It

ranges from granodiorite to quartz monzonite to granite. Marsh and Smith (1997) report data

-12 -15 from Hanson and Nash (1994) concluding that the stock crystallized at a high fO2 of 10 to 10

bars at 750°-850° T.

The Little Cottonwood stock is host to a few subsidiary intrusive units, including aplite

dikes, rhyolite dikes, and lamprophyre dikes, and two other small map units, the White Pine

intrusion and the Red Pine porphyry (Fig. 3). The White Pine and Red Pine units host the small

White Pine porphyry molybdenum deposit, which is found amidst more widespread quartz-

sericite-pyrite (QSP) alteration. The deposit is characterized by stockwork veins of quartz ±

sericite, pyrite, scheelite, and molybdenite, as well as veins of ribbon quartz and vug-filling quartz. Towards the southern margin of the White Pine, there is a breccia pipe which is Mo- mineralized in some locations (Sharp, 1958; John, 1989). It also hosts a pebble dike, which is a common occurrence in porphyry-type deposits (Bryner, 1961; Mutschler et al., 1981; Sillitoe,

1985).

Adjacent to the Little Cottonwood stock, and separated by the Fort Canyon Fault, is East

Traverse Mountain, a highly brecciated and faulted block comprised of Pennsylvanian Oquirrh

Group strata, Tertiary conglomerates, and argillic- and propylitic- altered Tertiary volcanic rocks

(Biek, 2005). There is recent evidence that East Traverse Mountain is a landslide block, not a fault-bounded horst, that was induced to slide subsequent to the intrusion of the Little

Cottonwood stock and the onset of normal faulting (Keith et al., 2017; Chadburn et al., 2018).

Figure 3. Geologic map of the White Pine Mo-W deposit and surrounding area. The Little Cottonwood stock is in pink, White Pine intrusion in light purple, Red Pine and Rhyolite phases in dark purple, and the area of intense stockwork quartz-sericite-pyrite alteration is in bluish-purple. Samples are shown as blue dots for White Pine, green for Red Pine, red for rhyolite dikes, and purple for Little Cottonwood stock. The pebble dike is present as a light blue dot.

METHODS

We spent three field seasons collecting samples and mapping in the vicinity of the White

Pine Mo-W deposit and in the East Traverse Mountains, producing a suite of representative samples of each lithology from the Little Cottonwood stock and East Traverse Mountain to show variations within units. We used the Rigaku ZSX Primus II X-ray fluorescence spectrometer at

Brigham Young University to determine major element and certain trace element concentrations of most samples. Major element oxides analyzed include SiO2, TiO2, Al2O3, Fe2O3, MnO, MgO,

CaO, Na2O, K2O, and P2O5, and were accomplished using fused glass disks. Trace elements

were analyzed using pressed powder pellets, and include Sc, V, Cr, Ni, Cu, Zn, Ga, Rb, Sr, Y,

Zr, Nb, Ba, La, Ce, Nd, Sm, Pb, Th, and U.

Samples selected for geochronological work were crushed in a roller mill to 30 mesh size, then passed through a magnetic separator. The heavy minerals in the non-magnetic portion were then separated using tetrabromoethane and methylene iodide. Zircon grains were selected by hand, mounted in epoxy, polished, and carbon coated. Cathodoluminescence (CL) images of the zircon epoxy mounts were acquired at Brigham Young University using a Gatan miniCL detector, and either a ThermoScientific Apreo C Low-Vacuum Scanning Electron Microscope or an ESEM XL30 FEI Scanning Electron Microscope. Images were recorded using a spot size of

14 µm, a tilt of 0°, a scan acquisition time of 6 ms/pixel, and an accelerating voltage of 15 kV on the Apreo, and spot size of 6 µm, tilt of -10°, scan time of 66 ms/line, and voltage of 15 kV on the ESEM. This was done in order to show zoning in the zircon grains, so as to avoid ablating inherited cores instead of the most recent magmatic rims. U-Pb zircon ages were determined at the Inductively Coupled Plasma-Mass Spectrometry Lab at the University of Utah Department of

Geology and Geophysics, using the methods of Couper (2016). The laser ablation system was a

193 nm Teledyne Photon Machines Analyte Excite, and the ICP-MS was either an Agilent

7500ce, or a Thermo Finnigan Neptune Plus. Laser settings were 15 kV and 6 Hz repetition rate, yielding a fluence of 9.51 J/cm2 at 100% beam energy. Helium was used as a carrier gas at 1

L/min. The zircon standards used were Plesovice (Sláma et al., 2008) and 91500 (Wiedenbeck et al., 1995). The round robin included these standards as well as r33 and Temora II (Black et al.,

2004), and SL2 (Gehrels et al., 2008). There were approximately 35-50 spots analyzed in each sample, most with a 20 micron spot size, while others were sampled with a 15 micron spot because of the small size of the grains. Data were reduced using Iolite 2.5 (Paton et al., 2011)

and ages extracted using IsoPlot (Ludwig, 2012) and IsoPlotR (Vermeesch, 2018). Typical

uncertainties are ± 2%, or about 600,000 years for these Oligocene rocks.

GEOLOGY

Zircon U-Pb Ages

Table 1 presents the calculated U-Pb ages for rocks from the Little Cottonwood magmatic system. Because the age ranges were larger than expected from analytical error alone

(as indicated by elevated MSWD values), we used the TuffZirc routine in IsoPlot 4.0 to estimate the crystallization ages from the concordant 206Pb/238U ages.

Table 1. Summary of LA-ICP-MS U-Pb zircon ages and uncertainties from samples in the White Pine Mo-W deposit and East Traverse Mountains.

The zircon age spectra reveal three broad peaks at about 29.7, 27.0, and 25.9 Ma, which

we interpret to be intrusion ages for the Little Cottonwood stock, the White Pine phase, and the

Red Pine Porphyry respectively (Fig. 4). The first two peaks are consistent with published ages

for the Little Cottonwood stock and for the White Pine phase (Constenius, 1998; Vogel et al.,

2001; Stearns et al., 2017; Smyk et al., 2018). However, the kernel density plots reveal other

peaks and shoulders that show each intrusive phase has inherited zircons from an older phase.

Moreover, most of the samples also have a younger peak that corresponds with the age for a younger intrusion. For example, CH-031 has a prominent peak at ~27.5 that we interpret to be zircons crystallized from the White Pine magma; it has another smaller peak at about 30.5 Ma that are probably xenocrysts (or antecrysts) from the Little Cottonwood stock. A third population of zircons in CH-031 lies at ~25.9 Ma, which we interpret to be zircon that either re-equilibrated at the time of Red Pine intrusion or newly crystallized from hydrothermal fluids associated with the Red Pine phase.

In order to understand the sequence of rocks that the East Traverse Mountain pebble dikes might have explosively sampled on their routes to the surface, we also analyzed 30 grains from the matrix of one dike from the East Traverse Mountains (CH-039). Concordant U-Pb zircon ages range from 28.5 Ma to as much as 1,613 Ma and are consistent with incorporation of detrital zircon with Proterozoic ages in Upper Proterozoic to Paleozoic clastic sedimentary rocks cut by the pebble dikes. There is a large cluster of zircons with ages ranging from 994 Ma to

1162 Ma; these are similar to the detrital zircon ages found in the Big Cottonwood and Little

Willow formations that underlie the Paleozoic sedimentary rocks (Spencer et al., 2012). The peak age found in the dikes is 30.5 Ma, within the known range for the Little Cottonwood stock, which extends from about 29 to 31 Ma (this paper, Smyk et al., 2018; Vogel et al., 2001). There is a smaller peak formed by 3 grains with ages of about 32.5 Ma. Perhaps, these zircons were derived from an unexposed precursor to the Little Cottonwood stock or from the 32 to 35 Ma solidified Alta Stock (Vogel et al., 2001; Smyk et al., 2018).

Figure 4. Probability plots of zircon ages from the White Pine Mo-W deposit and two samples from a pebble dike in the East Traverse Mountains. Ages were obtained for the Little Cottonwood stock, White Pine intrusion, Red Pine porphyry, and pebble dike clasts and matrix. Analyses by LA-ICP-MS at University of Utah using Plesovice zircon (Sláma et al., 2008) as the calibration standard. n is the number of analyses used in age calculation/acceptable analyses (<10% discordant). Ages from the nearby Alta stock and Clayton Peak stock of the Wasatch Igneous Belt are included for comparison, as reported by Vogel et al., 2001 and Smyk et al., 2018.

Unit Descriptions

Previous studies on the Little Cottonwood stock and the associated Mo-W deposit (Sharp,

1958; Crittenden, 1965; John, 1989; Hanson, 1996; Marsh and Smith, 1997; Vogel et al., 2001;

Smyk et al., 2018) noted the presence of multiple phases within the stock, including the White

Pine intrusion, the Red Pine porphyry, and several rhyolite and pebble dikes. These units are

11 described below using field, petrographic, and chemical characteristics. Secondary alteration may partially obscure the original igneous textures, mineral assemblages, and whole rock

chemical composition. Due diligence was taken to try to discriminate between primary igneous

and later hydrothermal alteration features, but this was not always possible.

This study attempted to give a thorough geochemical, textural, mineralogical, and

geochronological description of each unit, allowing for better mapping and future study of the

deposit. This has ultimately proved to be difficult for the White Pine intrusion and the Red Pine

porphyry, because of the wide variety of overlapping textures and geochemical trends, and

overprinting of hydrothermal alteration on both units. Positive identification seems to only be possible when porphyritic Red Pine rocks are encountered, or if a U-Pb age is obtained. For unit description purposes, all samples without a U-Pb age were assigned to the age-defined unit

which they most closely resembled, with the understanding that further geochronological studies

could cause some samples to be reassigned to a different unit.

Little Cottonwood Stock

The Little Cottonwood stock is a felsic granitoid with an exposed area of 115 km2 (Fig.

1). It ranges from coarsely phaneritic-porphyritic in the outer portion of the stock, with an equigranular groundmass, to the inner portion of the stock which has a finer-grained phaneritic to seriate groundmass (Marsh and Smith, 1997). It contains ellipsoidal mafic enclaves, which generally range from a few cm to nearly a meter across in the north-east part of the pluton, but

can be up to several meters across, and are commonly associated with mafic schlieren (Hanson,

1996). The Little Cottonwood stock is high-K, magnesian, and metaluminous (Marsh and Smith,

1997). When plotted on a total alkali-silica diagram, the composition clusters around the triple

point between granite, granodiorite, and quartz monzonite (Fig. 5). In the IUGS QAP diagram, it plots primarily as granodiorite, with fewer samples of quartz monzonite and granite (Fig. 6).

Figure 5. Chemical variation diagrams of the intrusive igneous units. Grey circles indicate highly altered samples. Little Cottonwood Stock is purple, White Pine is blue, Red Pine is green, and rhyolites are red. (A) total alkali-silica diagram (after IUGS). Note that Rhyolite is far more silicic than the rest of the units, and White Pine and Red Pine are generally more silicic than the Little Cottonwood Stock. (B) FeO/(FeO+MgO) diagram after Pearce et al. (1974)

of the intrusive units. The White Pine and Red Pine units have a higher Fe/Mg ratio, suggesting a lower fO2 in their respective source magmas than in the Little Cottonwood stock. The Rhyolites are ambiguous because of high Si. (C) Ti-Sc variation diagram. Note the trend in the rhyolites directly opposing the trend in the other intrusive units. (D) Ti-Sr variation diagram. Note that Sr decreases to quite low concentrations in the White Pine intrusion, and none of the units are especially enriched in Sr. (E) Rb-Y+Nb tectonic discrimination diagram after Pearce (1984). The intrusive units (except the rhyolites) generally fall into the Volcanic Arc setting, and the outliers with higher Rb are probably affected by alteration. (F) A-type granite discrimination diagram after Whalen et al. (1974). All but one sample fall in the I-type field, indicating a magmatic arc source, rather than rift-related or atectonic-sourced A-type granites typically associated with Climax-type deposits. (G) Ti-Zr variation diagram. This shows that the White Pine intrusion and the Little Cottonwood Stock form two discrete differentiation trends. The rhyolite is far more differentiated than any other unit.

Figure 6. Classification of the intrusive igneous units based on the IUGS QAP diagram. The Little Cottonwood stock is dominantly granodiorite, with some granite and a quartz monzonite; the White Pine intrusion is dominantly

granodiorite, with some tonalite and granite, and altered rocks that are outliers; the Red Pine is granodiorite to granite; the Pebble Dike clasts are dominantly granodiorite, with a few granite samples. Fields are abbreviated as follows: Qrg= quartz rich granitoids; Gr=granite; Grd=granodiorite; Tn=tonalite; Afg=alkali feldspar granite; Qm=quartz monzonite; Qmd=quartz monodiorite.

In most locations, K-feldspar megacrysts up to 6 cm long are present. The matrix

phenocrysts are comprised dominantly of plagioclase and commonly strained quartz, with lesser

amounts of K-feldspar. Plagioclase rim compositions are An22-24; perthitic K-feldspar, with

exsolved albite present as lamellae or small irregular blebs which are optically continuous in

some grains, were recalculated to a magmatic composition of about Or77-81 (Hanson, 1996).

Mafic phases include primarily biotite (Fe/(Fe+Mg) 0.38-0.47, Xphlogopite 0.53-0.63), usually greater than 5% in modal abundance, and minor amounts of euhedral amphibole which plots from hornblende to tremolite, and has Fe/(Fe+Mg) of 0.271 to 0.536. Accessory phases include close to 1% titanite, as well as allanite, magnetite, apatite, and zircon. There is no significant mineralization, and it is generally unaltered in hand sample, except along fractures near the

White Pine intrusion. However, thin sections show even the freshest samples have minor chloritization of biotite and some hydrothermal epidote.

Mafic enclaves hosted in the Little Cottonwood stock have the same mineral assemblage as the host rock, though with different modal abundances (Hanson, 1996). The enclaves are porphyritic-aphanitic; most enclaves are more than half poikilitic plagioclase, followed by biotite, hornblende, and small amounts of quartz and K-feldspar. Commonly hornblende has cores of biotite. Accessory minerals include up to 3% titanite, which is large and often euhedral, but some grains are highly included with small anhedral plagioclase, and some grains appear to have minor resorption. Other accessory phases include lesser amounts of apatite, Fe-Ti oxides, and zircon. In one sample, sprays of acicular apatite radiate from central points. Alteration phases include chlorite, epidote, and calcite. We interpret the enclaves to be formed by injection

and mixing of a more mafic magma. A preferred orientation of the rock fabric elements, complex

shapes, inclusions of K-feldspar megacrysts, association with mafic schlieren, and sprays of

elongate apatite are consistent with this idea, possibly being formed by shearing as hotter mafic

magma was introduced and then quenched in the cooler Little Cottonwood stock magma, or by

ballooning of the magma chamber that compressed the enclaves outwards (Dorais et al., 1990).

White Pine Intrusion

The White Pine intrusion occurs in the central northeastern portion of the Little

Cottonwood stock (Figs. 1 and 3). The inferred extent of the White Pine is mostly obscured by foliage, and Quaternary glacial, fluvial, and colluvial cover. Sparse outcrops occur along trails, in

more resistant domes in the center of the glacial valley, in the breccia pipe, and along the tops of

ridges. The contact of the White Pine intrusion with the Little Cottonwood stock is difficult to

recognize, both because of the similarity in appearance of the two phases in hand sample, and

because of widespread ground cover and pervasive overprinting of heavy QSP alteration. Sharp

(1958) described the contact as ranging from sharp to gradational. The only location where we

observed a sharp contact was a small region on the southern end of the outcrop. Our mapping

(Fig. 3) has extended the range of the White Pine outside that mapped by Sharp (1958) and

Crittenden (1965).

In its fresh form, which is uncommon, the White Pine intrusion is texturally and

mineralogically similar to the fine-grained facies of the Little Cottonwood stock which it

borders, just slightly more leucocratic (Table A1), with a medium- to fine-grained phaneritic or

seriate groundmass, and occasional poikilitic K-feldspar megacrysts (Fig. 7). The matrix

phenocrysts are dominantly plagioclase, with lesser amounts of quartz and still less K-feldspar.

The quartz phenocrysts tend to be larger than plagioclase (up to 1 cm), and in some samples are

square and stand out in prominent relief. The only mafic phase is biotite, and this is usually less than 5% in modal abundance; unlike in the Little Cottonwood stock, hornblende is lacking. It

also has only trace amounts of titanite, which is usually not visible in hand sample. White Pine

titanites have a distinct geochemistry from the Little Cottonwood stock, with different Fe/Al

ratios (Martin et al., 2018). Other accessory mineral phases include apatite, allanite, zircon, and

possibly Fe-Ti oxides. Significantly, the White Pine intrusion consistently lacks mafic enclaves,

unlike the encompassing Little Cottonwood stock.

Figure 7. Photomicrographs of White Pine intrusion and Red Pine porphyry. All photos taken at 2x magnification.(A) Typical seriate texture in the White Pine, with quartz, plagioclase, K-feldspar, and biotite. Note secondary sericite, both fine-grained on the plagioclase, and coarser grained flakes. Fe oxides, and secondary pyrite, clays, and epidote are also visible. (B) A pocket of fine-grained interstitial groundmass from the White Pine, comprised mostly of quartz, with some K-feldspar. (C) Mineralized White Pine. Note angular quartz and secondary K-feldspar, and abundant sericite plates. This type is the result of the most intense QSP and potassic alteration. (D) Red Pine-porphyritic variety. Note the resorbed quartz grain, and the very fine-grained groundmass, comprising ~50% of the rock. Sericitic alteration is pervasive. (E) CH-053, the only known phaneritic example of Red Pine. Similar in texture and mineralogy to White Pine intrusion, but somewhat smaller grain size.

The White Pine intrusion is usually moderately to heavily altered with quartz + sericite +

pyrite. In outcrop, it almost universally weathers orange from limonite and jarosite formed by

alteration of primary sulfides, which permeate fractures in the rock, especially in the stockwork.

The fractures are lined with pyrite and sericite, which are also present disseminated through the

rock, and with quartz. Sometimes pyrite occurs in small clusters surrounded by splotchy red

alteration patches several cm wide. White Pine rocks often have a fine-grained interstitial

groundmass of quartz ± K-feldspar, up to about 15% volume (Table A1). It is not clear whether

this groundmass is magmatic or due to post-magmatic silicification, or both, although fine-

grained feldspars in some patches of groundmass suggests quenched melt (Fig. 7). Plagioclase is

usually heavily altered to sericite ± clays, especially the cores, and biotite is often partially to

completely altered to sericite and/or chlorite, with inclusions of alteration rutile. K-feldspar generally has only minor clay alteration, even when biotite and plagioclase are completely replaced. Smaller K-feldspar grains appear more readily altered than essentially fresh megacrystic

K-feldspar. The disseminated pyrite often is weathered to limonite, sometimes with a thin rim of limonite around otherwise fresh pyrite. Sericite, in addition to replacing plagioclase and biotite,

is present as euhedral tabular blades or fans. In addition to the QSP alteration assemblage, there

is also hydrothermal epidote, monazite, barite, rutile, and uranopolycrase

((U,Th)(Ti,Ta)2O6) identified via electron microprobe, and possibly other oxides from late-stage

weathering.

The White Pine is Mo-mineralized in a few small occurrences, including the breccia pipe

and the west ridge, as revealed in mine adits. This variety is more altered along fractures, and can

often be recognized in outcrop by its weathered color, which is more yellowish than the reddish color it has when only QSP-altered. The mineralized White Pine is heavily potassically and QSP-

20 altered, primarily comprised of white to off-white sericite, very euhedral K-feldspar megacrysts,

and quartz, though in some instances it is silicified and is composed mostly of quartz + sericite,

with few feldspars. Plagioclase is a minor phase, usually present only in trace amounts, and has

probably been replaced. Titanite, allanite, apatite, and zircon are absent or present only in trace

amounts. The mineralized rock is seriate to phaneritic in texture, though the original magmatic

texture is mostly destroyed (Fig. 7). It hosts molybdenite, sphalerite, and pyrite, trace barite and

likely hydrothermal monazite (Schandl and Gorton, 2004), and several tens of percent of coarse-

grained tabular sericite. It commonly hosts large clots or cubes of pyrite up to 1 cm across, sometimes fresh and sometimes weathered out, leaving void spaces. It also hosts veins of

molybdenite, veins of alteration clay and oxides, and quartz veins. Because of alteration, there

are no mafic phases present.

The chemical fractionation trends in the White Pine magma are essentially the same as in the Little Cottonwood stock (Figs. 5 and 8), and the only major mineralogical differences

between the White Pine and Little Cottonwood stock are the lack of hornblende, and less than

half as much modal titanite (Table A1). The White Pine is magnesian, high-K (Marsh and

Smith, 1997), and is slightly more silicic than the Little Cottonwood stock, plotting as granite in

a total-alkali silica diagram, although there is much overlap of the two intrusive units (Fig. 5).

The White Pine is compositionally different from the Little Cottonwood stock, having higher

FeOT/(FeOT+MgO) for a given SiO2 concentration, and having a separate TiO2-Zr trend, with higher Zr for a given TiO2 concentration (Fig. 5).

Figure 8. Chemical variation diagrams of the intrusive igneous units, showing major elements vs. SiO2. TiO2, Al2O3, FeOT, MnO, MgO, CaO, and P2O5 vary inversely with SiO2 across all units. Grey circles indicate highly altered samples. Little Cottonwood Stock is purple, White Pine is blue, Red Pine is green, and rhyolites are red. Na2O slightly decreases and K2O slightly increases with increasing SiO2, but both show some scatter. The rhyolites are invariably at the extreme end of all these trends, and the White Pine and Red Pine are just slightly more evolved than the Little Cottonwood stock. Red Pine Porphyry

The Red Pine porphyry, first recognized by Sharp (1958) but ignored by subsequent

researchers, includes two texturally and chemically distinct rock types of the same age, found in

a few small scattered outcrops, as well as a few instances of float (Fig. 3). Because the Red Pine is usually heavily altered and obscured by glacial deposits and overgrowth, contacts with the

White Pine intrusion are difficult to find, and consequently, the boundaries of the Red Pine

porphyry are largely inferred. Widely spaced outcrops of Red Pine occur on the east ridge of the

valley and on the west ridge separating White Pine and Red Pine canyons, and in one outcrop on

the north end of the deposit (Fig. 3). Therefore, the Red Pine is interpreted to be a separate magma that intruded the mostly solidified White Pine as small, widely spaced bodies. Most Red

Pine compositional trends are indistinguishable from those of the White Pine (Figs. 5 and 8).

Thus, textural variations of the Red Pine porphyry may be the best way to distinguish it from

other units.

The porphyritic variety of the Red Pine is dominantly a quartz porphyry with rarely any

visible phenocrysts besides quartz. It is quite porphyritic, with about 50% fine-grained white to

tan groundmass of plagioclase, K-feldspar, and quartz. It has phenocrysts of plagioclase (some

with rounded edges, but otherwise not resorbed), prominent phenocrysts of square, often resorbed/embayed quartz grains up to 5 mm across (Fig. 7), as well as K-feldspar (locally

poikilitic and megacrystic), and relict biotite. Alteration is intense; there are no surviving mafic

phases or Fe-Ti oxides, phenocrystic plagioclase is almost completely altered to clays and

sericite, and pockets of clay minerals are common. The groundmass is altered beyond

23 recognition. Only quartz and K-feldspar megacrysts are unaltered. Biotite is completely altered

to sericite and Fe-Ti oxides, especially rutile, ± chlorite. There is no visible titanite or apatite,

and allanite and zircon are minor. Cubes of pyrite 0.5 cm in size, often weathered out to leave

cubic limonite-rimmed void spaces, are very prevalent.

The phaneritic variety of the Red Pine porphyry bears little textural resemblance to its

porphyritic variety (Fig. 7), based on its only known sample CH-053. In hand sample it is

phaneritic, light-grey to white in color, with small plagioclase and biotite grains, and sparse larger phenocrysts of quartz and K-feldspar. Accessory minerals include apatite, zircon, titanite, allanite, and oxides. It is rather fresh, with minor secondary pyrite, sericite, epidote, and chlorite.

The texture is seriate, generally finer-grained than the White Pine or Little Cottonwood stock,

and most phenocrysts, including quartz and the feldspars, are subhedral to anhedral. It lacks clots

of very fine-grained groundmass. There appear to be two populations of plagioclase: many that

are completely fresh, and others with altered cores of various sizes. The outcrop from which this

rock was sampled is also weathered yellow-orange on exposed surfaces.

Pebble Dikes and Breccia Pipe

Pebble dikes are phreatomagmatic features formed when meteoric and/or magmatic water

in low-permeability rocks is superheated and flash-boiled, causing brecciation and the opening of

fractures, in some cases all the way to the surface. As the super-heated water vents upward, it

entrains clasts of brecciated wall rock, and the slurry is emplaced in the overlying rock column.

Clasts may be rounded via abrasion or thermal spallation (Sillitoe, 1985; Johnson, 2014).

Sharp (1958) described a narrow (~0.5 m) near-vertical pebble dike cutting the White

Pine porphyry with inclusions of the Little Cottonwood stock and of the leucocratic White Pine

intrusion. We found this dike, which strikes east-west, near the Mo-mineralized breccia pipe

(Fig. 3). The intrusive rock hosting the pebble dike is hydrothermally altered, but not Mo-

mineralized. The matrix of the dike is brownish to greyish green, and comprised of broken phenoclasts of biotite, quartz, plagioclase, K-feldspar, and finely comminuted material, as well as various alteration phases and calcite. Rounded to subrounded intrusive igneous clasts (<15 cm in diameter, most commonly about 1-3 cm in diameter, but some as small as a few mm) make up

25-50% of the rock. In some hand samples the clasts are elongated and have a preferred orientation. Based on texture and mineral assemblage, Sharp (1958) identified both White Pine and Little Cottonwood stock clasts. Most of the clasts from this pebble dike examined for this study are White Pine or Red Pine; if Little Cottonwood stock clasts are present, they are a minor constituent of the intrusive clast population. There are smaller (5-10 mm) quartz clasts included as well, with an abundance of only about 5-10% of the rock. These are apparently mostly phenoclasts from brecciated intrusive units, or hydrothermal vein quartz; evidence for sedimentary clasts is equivocal. Pyrite is present as larger rounded grains in some clasts, and smaller, more numerous grains in the matrix. The level of alteration of clasts in this pebble dike varies widely, based on the sericitic alteration of plagioclase and biotite, and chloritic alteration of biotite; some of the pyrite was probably introduced during alteration. The pebble dike includes both fresh and altered clasts and is itself somewhat altered. This probably means the dike occurred syn-mineralization of the White Pine intrusion or Red Pine porphyry.

Other magmatic-hydrothermal features that may occur in porphyry deposits in addition to pebble dikes are breccia pipes (Thomas and Galey, 1982; Taylor et al., 2012). The White Pine intrusion contains one such breccia pipe (Sharp, 1958; Krahulec, 2015; Smyk et al., 2018), which is possibly part of the same phreatomagmatic system. It is exposed less than 100 m north of the

pebble dike in the White Pine phase. It forms the focus of Mo mineralization in the White Pine

deposit, and is roughly oval and about 200 m across. Its peripheral part contains angular

fragments of White Pine <16 cm in diameter enclosed in a highly sericitized matrix with

molybdenite, and sometimes cemented by pyrite- and molybdenite-bearing vuggy quartz (Smyk et al., 2018). The molybdenite concentration is higher near the margins of the breccia, and occurs as aggregates around clast margins, (Krahulec, 2015) and as veins in the potassically altered

White Pine clasts. Coarse-grained “bull” quartz locally envelopes the breccia zone, marking its boundary with a zone of weakly mineralized quartz stockwork veins. The breccia and the stockwork zones include QSP alteration along with molybdenite, galena, sphalerite, and a little scheelite. Minor fluorite-sericite zones formed slightly later (Sharp, 1958).

Numerous pebble dikes also cut Oligocene volcanic rocks at the crest of the East

Traverse Mountains about 17 km away (Fig. 1) (Hoopes et al., 2014). This study found that these are quite similar to those in the White Pine intrusion. Although some dikes are visible in natural exposures and recent trenches, evidence of pebble dikes is generally found as float. The few

outcrops that are visible range from 30-50 cm in width, mostly close to 30 cm. The sparse nature

of the float and recent development of the region makes it difficult to reconstruct dike length and

orientations. They generally trend northwest-southeast, and can only be confidently traced for a few m to approximately 100 m, though they could potentially have much greater lengths. There is no consistent evidence for wall rock alteration.

The matrix of the dikes is invariably very fine-grained, and ranges from light gray to tan to black. The matrix includes broken phenoclasts of quartz, plagioclase, K-feldspar and biotite, along with grains of pyrite. Clasts are generally angular, but include some well-rounded quartzite pebbles. They vary in size from smaller than 1 cm to up to around 10 cm in diameter, but are

most commonly 0.5-2 cm, and can comprise between 10-80% of the rock, but generally make up

40-50% of the rock. The clasts include quartzite, extrusive intermediate composition rocks, pockets of hydrated clay, possible volcaniclastic sedimentary rocks, and significantly, felsic clasts of coarse-grained intrusive igneous rock. It is not clear what the origin of the clay pockets is; they could be bleached shales from the underlying Manning Canyon Formation, or highly argillically altered igneous clasts. In most dikes quartzite is the most common clast type, while in other samples extrusive volcanic clasts are the most common type. Some pebble dikes have intrusive and sedimentary clasts, while others only have sedimentary clasts (Fig. 9). Even within one thin section the intrusive igneous clasts can be of varying rock type. Based on texture and mineral assemblage, we have correlated the intrusive clasts with all three of the intrusive phases mapped 17 km to the east in the White Pine deposit. The intrusive clasts vary between nearly fresh to heavily QSP altered, and sometimes have alteration selvages around their rims.

Alteration of the pebble dikes in hand sample is evident from the brownish-red to yellowish alteration staining on the surfaces of most pebble dikes as well as rimming many of the clasts. This is probably jarosite, crandallite, and iron oxide after hydrothermal pyrite, and other alteration phases from argillic alteration. Argillic alteration is especially apparent in the altered volcanic clasts, and as yellowish clays on some faces. Fresh rock fractures show un-oxidized pyrite. Some of the pebble dikes are silicified, break in a conchoidal manner, and host multiple generations of thin silica veins. It is unclear to what extent alteration of the dikes occurred post- emplacement; most dikes show little to no evidence of post-emplacement QSP alteration or silicification besides possibly unrelated late-stage silica veins. Quartz, sericite, and pyrite often present in the matrix may have been sourced from previously altered intrusive rocks pulverized by pebble dike emplacement. Some pebble dikes don’t appear to have undergone QSP alteration

at all, but appear fresh in hand sample and in thin section. Accessory/alteration phases in pebble

dike mineral separates identified via SEM analysis include molybdenite, barite, uranothorite,

arsenopyrite, scheelite, titanite, and possibly sanbornite (Ba(Si2O5)).

Figure 9. (A) Traverse Mountain pebble dike, with QSP-altered intrusive igneous clast on the right, and matrix on the left, bearing various sedimentary clasts. (B) Traverse Mountain pebble dike, showing a fresh Red Pine- Porphyritic clast on the left--devoid of any clay or sericite--and a possibly silicified Little Cottonwood stock clast on the right, with a matrix of phenoclasts and comminuted material in between. (C) Another view of the same porphyritic clast as in (B). Note quench textures and resorbed quartz. All photomicrographs taken at 2x magnification. The sedimentary rock clasts are similar to quartzites of the Oquirrh Group (Biek, 2005),

consisting of light brown to tan fine-grained quartzite or sandstone, and black chert. Thick

sections of these late Paleozoic units are found on the flanks of the intrusion and are inferred to

have overlain the plutonic complex at the time of emplacement of the pebble dikes. There appear

to be multiple generations of pebble dikes in East Traverse Mountain, since at least one pebble

dike is cross-cut by another one.

The pebble dikes in the East Traverse Mountains and in the White Pine intrusion appear

to be part of the same magmatic-hydrothermal system. Intrusive igneous clasts in the East

Traverse Mountain dikes have the same texture and mineralogy in thin section as the White Pine,

porphyritic Red Pine, and Little Cottonwood stock units (Figs. 7 and 9). Uranothorite,

molybdenite, and scheelite in the Traverse Mountain dikes are indicator minerals of

molybdenum deposits, and are also found in the White Pine deposit. There is wide scatter in the

ages of the two pebble dike samples taken from Traverse Mountain, but there are zircon

populations in both that were of White Pine/Red Pine age, as well as of Little Cottonwood stock

age (Fig. 4 and Table 1). This is all evidence that the Little Cottonwood stock pebble dikes were

in the lower part of the system, which only sampled intrusive rocks, while the East Traverse

Mountain pebble dikes which cut surficial volcanic rocks were the upper portion of the same

system, which sampled intrusive rocks as well as the overlying sedimentary rocks.

Rhyolite Dikes

Rhyolite dikes (which Sharp (1958) called “aplite dikes”) are white, off-white, light grey, or tan. They are generally oriented E-W, usually less than a meter white, and are visible mostly as float, though in the south, west, and north-east margins of the White Pine intrusion they occur in outcrop, mostly 1-2 m in width, and one rhyolite dike in the northeast part of the deposit is about 10 m in width. The homogeneous aphanitic matrix is made of quartz, K-feldspar, plagioclase, and fine-grained secondary sericite and clays, and comprises 95-99% of the rock.

Fracture faces are typically broken in a conchoidal manner, and weathered faces and fracture

29 faces commonly have red-brown pyritic alteration, and sometimes black dendritic mineral

growths. Square quartz phenocrysts, up to 0.5 cm in size, appear gray in comparison to the

groundmass. These rocks have moderate to heavy QSP alteration, as shown by the high silica

content, with some >80% SiO2. A few samples are cut by discrete quartz veins. In addition to

fine-grained quartz and quartz veins, alteration phases usually include small disseminated pyrite

cubes commonly 1 mm in size, but up to 0.5 cm in diameter, mostly weathered away to leave

squarish oxide-lined void spaces, and sericite flakes less than 1 mm in size that permeate the

matrix and replace minor relict feldspar phenocryst. The accessory minerals in the other units

(apatite, titanite, and zircon) have not been found in the rhyolite, even in mineral separates.

Chemically, the rhyolite dikes are much more differentiated than the other units, being highest in silica and incompatible K2O, Rb, Nb, Y, Th, and U, and lowest in compatible TiO2,

FeOT, Al2O3, MgO, CaO, P2O5, Sr, and Zr (Figs. 5, 6, and 10, and Table A2). They also have no

surviving ferromagnesian silicates, but that could partly be due to the alteration. Nb anomalies

(Fig. 10) are obscured by low LREE concentrations, which are probably the result of

fractionation of LREE- enriched allanite ± monazite. Positive Pb and negative TiO2 anomalies

are present.

The ages of the rhyolite dikes relative to the pebble dikes and breccia pipe are not clear

from cross cutting relations, and an attempt to separate zircons for geochronology was

unsuccessful. However, the roughly east-west orientation, and nature of the secondary alteration,

both suggest that the rhyolite dikes were part of the same White Pine or Red Pine magma system

that was subsequently altered under conditions that stabilized the quartz + sericite + pyrite

assemblage.

30 Figure 10. Spider diagram of all intrusive plutonic units, normalized to primitive mantle. Purple is Little Cottonwood Stock, blue is White Pine, green is Red Pine, and red is rhyolite. Note the negative Nb, P, and Ti anomalies, and the positive Pb anomaly. Highly altered samples were excluded.

Geochemistry

As a whole, the Little Cottonwood stock and the White Pine-Red Pine phases are calc-

alkaline (Fig. 5) and compositionally similar to other volcanic and plutonic rocks from the

adjacent Great Basin (e.g., Christiansen and McCurry, 2008). Major element SiO2 variation

diagrams (Fig. 8) show relatively straightforward trends in the freshest rocks, with TiO2, Al2O3,

FeOT, MnO, MgO, CaO, and P2O5 varying inversely with SiO2 across all units. Na2O slightly

decreases and K2O slightly increases with increasing SiO2, but both show some scatter. The

rhyolites are invariably at the extreme end of all these trends.

Hydrothermal alteration has affected the more mobile elements. Rb and K2O have a

linear positive correlation, and are moderately enriched, while Na2O and CaO both have a linear

negative correlation with K2O, and extend to very low concentrations in the most altered samples

of all units (Fig. 11) . Because of this, less-mobile trace elements have been used to study the pluton. In particular, Zr and TiO2 are helpful (Fig. 5), showing two separate and parallel positive

trends between these two immobile elements in the Little Cottonwood stock and in the

overlapping White Pine and Red Pine units. The rhyolites are far more depleted than the other

rocks in both of these compatible elements. Y varies negatively to quite low levels with SiO2,

although with some scatter. However, Y is enriched in the rhyolites (Figs. 10 and 12).

Figure 11. Variation diagrams showing alteration in mobile elements. Grey circles indicate highly altered samples. Little Cottonwood Stock is purple, White Pine is blue, Red Pine is green, and rhyolites are red. (A) Rb-K variation diagram, showing good positive correlation between the two elements, and a general cutoff boundary between unaltered samples and those with introduced K and Rb. Line shows cutoff between normal K and K-enriched

samples. (B) CaO-K2O variation diagram. The line shows trend of Ca-leaching due to alteration. (C) NaO-K2O variation diagram, showing the general cutoff boundaries between relatively unaltered samples and those that have had significant Na-leaching.

A normalized trace element diagram (Fig. 10) shows strong similarities between the

Little Cottonwood stock, White Pine intrusion, and Red Pine porphyry. These three units all have

strong positive large ion lithophile and Pb anomalies, and negative Nb and Ti anomalies. The

rhyolites have even higher Pb and U anomalies, very low Ba, P, and Ti anomalies, and a small

negative Sr anomaly. Moreover, the rhyolites have low concentrations of LREEs, but are

enriched in Y.

The variation diagrams of each unit indicate that the chemical trends were controlled by

fractional crystallization of the observed minerals from different parent magmas. Here SiO2,

K2O, Rb, and U behave incompatibly, and TiO2, Al2O3, FeOT, MgO, CaO, P2O5, Sr, Y, Zr, and

Ba behave compatibly. Most of the differentiation trends are typical of an I-type calc-alkaline

system (Marsh and Smith, 1997). Marsh and Smith (1997) proposed that the negative trends of

FeOT, MgO, TiO2, Zr, and CaO vs SiO2 are driven by fractionation of biotite, hornblende, titanite, magnetite, and zircon, while depletion of Sr is driven by plagioclase fractionation, and

depletion of Ba by biotite and K-feldspar removal. P2O5 depletion is likely due to apatite

fractionation.

An unusual characteristic of the igneous rocks in this area is their very low Y

concentrations, mostly below 10 ppm (Fig. 12) in all the intrusive igneous units except for the

rhyolite dikes. When plotted on a Sr/Y-Y diagram (Fig. 12), most of these samples fall into the

“adakitic” range of Sr/Y ≥20 and Y ≤18 (Defant and Drummond, 1993; Richards and Kerrich,

2007).

Average zircon saturation temperatures (Watson and Harrison, 1983) for the fresh rocks

are 768° for the Little Cottonwood stock (n=15, σ=15), 798° for the White Pine intrusion (n=17,

σ=10), 813° for the Red Pine porphyry (n=3, σ=16), and 729° for the rhyolites (n=6, σ=17), which are compromised by high K/Na ratios (Fig. 11). The zircon saturation temperatures of the

White Pine and Red Pine are distinctly higher than that of the Little Cottonwood stock, despite their generally higher SiO2 content and lower mafic mineral abundance.

Figure 12. Y variation diagrams. Grey circles indicate highly altered samples. Little Cottonwood Stock is purple, White Pine is blue, Red Pine is green, and rhyolites are red. (A) Si-Y variation diagram, showing a general depletion of Y with differentiation, except for the higher-Y rhyolites. (B) Sr/Y-Y diagram, showing that there is a distinct trend towards an “adakitic” signature within the White Pine, primarily due to low Y.

DISCUSSION

Origin and Evolution of the Magmatic Units

Tectonic Setting

The composition and age of the magmas that generated the Wasatch Intrusive Belt are

consistent with formation in a continental magmatic arc generated by subduction and rollback of

the Farallon plate. A subduction zone source of the Wasatch Intrusive Belt is supported by the

Little Cottonwood stock and White Pine compositions, which fall in the Volcanic Arc Granite

field in a Pearce et al. (1984) tectonic discrimination diagram (Fig. 5). Some of the more altered

Red Pine and White Pine samples do not fall into that field, but we interpret that as being due to

hydrothermal alteration enriching the rocks in soluble Rb. Enrichment in large ion lithophile

elements over high-field strength elements, such as a positive Pb anomaly and negative Nb

anomaly (Fig. 10), is another indicator of a subduction zone setting (Marsh and Smith, 1997).

Finally, low FeO/(FeO+MgO) ratios (Fig. 5), indicative of relatively high fO2 which stabilized

magnetite in the Little Cottonwood stock, and possibly in the unaltered White Pine and Red Pine

magmas, supports this idea, since subduction zone magmas typically have high fO2 (Wood et al.,

-12 -15 1990). Marsh and Smith (1997) report that the stock has an fO2 of 10 to 10 bars at 750°-

850°, in the QFM buffer range. The slightly higher FeO/(FeO+MgO) ratios, higher TiO2, and presence of minor ilmenite in the younger White Pine and possibly Red Pine, are probably due to a slightly lower fO2 for these somewhat younger and hotter magmas (798° and 813° in White

Pine and Red Pine, respectively, vs 768° in the Little Cottonwood Stock).

Although Vogel et al. (2001) proposed that an inherited weakness in the lithosphere along

a Proterozoic suture zone was responsible for the Wasatch Intrusive Belt, further study has cast doubt on the location of the suture there (Nelson et al., 2011). An alternative explanation could be a tear in the subducting Farallon plate. This would allow both flux melting and decompression melting of the mantle wedge along a linear trend, which would provide the source of the magma in the Wasatch Intrusive Belt, either as a heat source to induce melting of the crust, or as a source

of primitive magma followed by assimilation of continental crust (Best et al., 2016).

Isotopic data also point to a subduction zone setting. Farmer and DePaolo (1983) did Rb-

Sr and Sm-Nd isotope work on granites throughout the Great Basin, including the Little

87 86 Cottonwood stock. They gave an initial Sr/ Sr value of 0.70837 at 30.0 Ma, and ƐNd of -18.3 at the time of emplacement. Vogel et al. (2001) reported Sr isotope data with initial 87Sr/86Sr

ranging from 0.70736 to 0.70823, a range slightly below the value reported by Farmer and

DePaolo (1983). Smyk et al. (2018) reported an ƐNd of -17.7 for the Little Cottonwood stock,

which is similar to that of Farmer and DePaolo. They also reported the first Nd isotopic data

from the White Pine phase, with an ƐNd of -18.5.

This isotopic data from the Little Cottonwood stock and White Pine intrusion (Farmer

and DePaolo, 1983; Vogel et al., 2001; Smyk et al., 2018;) show magmas dominated by crustal

or metasomatized mantle lithosphere isotopic composition, but likely with a mantle component

too. Large amounts of regional magmatism from that time period argue against solely crustal-

derived magma generated by decompression during crustal extension. In comparison, intrusive

rocks from the Bingham Canyon porphyry Cu-Au-Mo deposit 25 km to the west, around 38 Ma

in age, have a range of 87Sr/86Sr ratios from around 0.707 to 0.709 (Fig. 13), and are interpreted as having some mantle component (Maughan et al., 2002). The presence of the Little

Cottonwood stock and White Pine intrusion at similar 87Sr/86Sr values (roughly 0.707-0.708)

may also signify some mantle input, followed by assimilation and contamination, which masked

the mantle signature. In contrast, late Cretaceous peraluminous granites--thought to be derived

exclusively from Proterozoic basement rocks in eastern Nevada--have much higher initial

87 86 Sr/ Sr values, ranging from about 0.710 up to 0.733 (Wright and Wooden, 1991). ƐNd values of roughly -17.5 to -18.5 in the Little Cottonwood pluton, which are about the same as the mantle-

influenced Bingham Canyon plutonic rocks, are another isotopic indicator of mantle input.

Figure 13. Sr and Nd isotope diagram of rocks from the Bingham Canyon complex, after Maughan et al., 2002. The black box encompasses the range of isotopic compositions of Little Cottonwood stock and White Pine samples from Vogel et al., 2001, and Smyk et al., 2018. In addition to isotopic data, major element whole rock data argue for a mantle

component. Partial melts of the crust are unlikely to be as mafic as some of the rocks in the

Wasatch Igneous Belt and the nearby Bingham Canyon pluton. For example, rocks from the

Clayton Peak stock in the belt plot as mafic as trachyandesite, basaltic trachyandesite, and basalt,

with as low as 55% SiO2 (Hanson, 1996; Vogel et al. 2001; Smyk et al., 2018), and the nearby,

slightly older Bingham Canyon complex has shoshonite down to 54% SiO2, and melanephelinite

as mafic as 44% SiO2 (Maughan et al., 2002).

Smyk et al. (2018) propose, based on trace element geochemistry, that the primary melt

of the Wasatch Igneous Belt is mantle derived, but underwent a large degree of crustal

assimilation in the thickened continental crust. They also propose that mafic lamprophyre dikes

found in the Little Cottonwood stock are evidence of underplating by a mantle-derived melt

which acted as a heat source to keep the system partially molten longer than if it was a closed

system. We concur with this notion, and suggest that mafic magma was generated in the mantle

above the subducting plate as it rolled back in the Oligocene.

Origin of the White Pine and Red Pine Units

We interpret the White Pine intrusion and Red Pine porphyry as being two magmas

derived separately from the Little Cottonwood magma, not differentiated from it. Because of the

lack of amphibole in the White Pine and Red Pine units, and the lower amount of biotite (Table

A1), the parental magmas might have initially had a lower content of Ca, Fe, and Mg attesting

to their slightly more “evolved” composition. Trace element variation diagrams, especially

TiO2-Zr (Fig. 5), show two overlapping differentiation trends of the White Pine and Red Pine

that are parallel to but separate from the Little Cottonwood stock. Also, the White Pine and Red

Pine units have higher FeO/(FeO+MgO) ratios for a given SiO2 concentration than the Little

Cottonwood stock (Fig. 5), which we interpret to indicate that that magma had a lower fO2. The

White Pine and Red Pine magmas extend to higher silica concentrations, and yet have higher zircon saturation temperatures than the Little Cottonwood stock (798° and 813° vs. 768°).

Finally, both units are a few m.y. younger than the Little Cottonwood stock, and they lack mafic

enclaves. Because of these differences, we propose that the White Pine intrusion and Red Pine

porphyry comprise distinct batches of magma that formed and then intruded separately from

each other and from the main phase of the Little Cottonwood stock. Their common center of

emplacement may be the result of spatiotemporal magmatic “focusing” along an already

weakened channel through the crust, as outlined for nested Sierra Nevada plutons by Ardill et al.

(2018)

38 The probability plots of the zircon ages for the White Pine and Red Pine magmas

contribute to this story as well (Fig. 4). Shoulders or peaks of older age in the White Pine and

Red Pine show that they likely incorporated some zircons from intrusions that pre-dated each of

them. Furthermore, shoulders or small peaks of younger age in the Little Cottonwood stock and

White Pine samples show possible influence on the zircon populations of these units by subsequent intrusions. The Mo-mineralized White Pine sample, CH-031, shows a prominent example of this, with older inherited zircons (~30.5 Ma) from Little Cottonwood stock, zircons that crystallized from the White Pine magma (~27.7 Ma), and an irregular young peak for secondary zircon (~26 Ma) that formed when the Red Pine magma was intruded. This appears to indicate that intrusions of Red Pine magma were the source of the Mo mineralization in the

White Pine intrusion.

Although the ages are different, the inferred mineralogy and chemical composition of fresh White Pine and Red Pine rocks and the textures of the non-porphyritic variety are quite similar. Thus, it is difficult to positively assign a sample to a specific geologic unit unless it has been dated, or has a porphyritic texture.

Y and the “Adakitic” Signature

The low Sr/Y ratios and Y concentrations in the Little Cottonwood stock and related units

(Fig. 12) are puzzling, as these are among the identifying characteristics of adakitic rocks. This is

significant, because a true adakitic slab-melt source for the pluton would require reinterpreting

the tectonic setting of the region. It is also significant because some workers have suggested a

link between adakites and porphyry-type mineralization (Defant et al., 2002), though others

dispute that (Richards and Kerrich, 2007; Richards, 2011). The issue is fraught with debate,

including what constitutes a truly adakitic signature (Richards and Kerrich, 2007; Castillo,

2012). For example, Smyk et al. (2018) suggest the Little Cottonwood stock is adakitic because

of high La/Yb and La/Y ratios which they interpreted to be caused by “deep melting of granulite

facies” metamorphic rocks within thick crust. However, rather than being a product of slab

melting, or melting a garnet-rich plagioclase-absent eclogite (as implied by Smyk et al., 2018),

the low-Y concentrations in the Little Cottonwood stock could be due to fractionation of certain

mineral phases (Moyen, 2009; Richards, 2011; Castillo, 2012). Amphibole, apatite, allanite, and

titanite, which are all found in the Little Cottonwood stock, preferentially incorporate Y when

they crystallize. In the Fish Canyon Tuff dacite, Y has partition coefficients of 13.5 (hornblende),

633 (titanite), and 181 (zircon) (Bachmann et al., 2005). The DY=162 in apatite in peraluminous

granite (Bea et al., 1994), 28.8 in allanite in low-silica rhyolite and 11.3 in amphibole in dacite

(Ewart and Griffin, 1994). Ackerson (2011) found partition coefficients of Y in titanite in various dacitic to rhyolite ignimbrites to vary from 490 to 1920 (with an outlier), with a geometric mean of 1247.

There is a general decrease in Y with increasing SiO2 in the Little Cottonwood stock, the

White Pine intrusion, and the Red Pine porphyry (Fig. 12), with partially overlapping fields. The fractionation of the above mineral phases, in particular titanite, would cause the residual liquids to be progressively depleted in Y. In at least one Little Cottonwood stock sample, these minerals combined make up more than 3.5% of the rock. Assuming titanite is the only phase important for

Y variation, a drop from 17 ppm to 7 ppm Y would only require about 20% crystallization (using our average modal abundance of 0.7% and an Y partition coefficient of 900). Similar calculations on the White Pine and Red Pine phases show that a much larger degree of

crystallization, up to about 80%, is required to reduce Y from 13 ppm to 1.5 ppm. This is a large

amount of fractionation, but the range in Sr concentration in the White Pine intrusion, decreasing

from 650 ppm down to below 200 ppm (Fig. 5, Table A2), supports extreme fractionation in the

evolution of the younger granitic magmas. Indeed, the low concentrations of Sr argue against the

deep crustal melting model, which requires the absence of feldspar in the source and

consequently high Sr concentrations in the melts. Thus, melting of garnet-bearing and feldspar

absent rocks (i.e., eclogites) is necessary to acquire the high Sr/Y ratios of true adakites but

appears to play no role in these low-Sr rocks. Moreover, since the low Y concentrations are not

an indication of a relationship with adakite, the notion that the Little Cottonwood stock is

“fertile” for ore mineralization is not supported by our data (cf., Smyk et al., 2018).

Rhyolite Dikes

The rhyolites are enigmatic because their composition is so different than the other units

in the composite pluton: they have higher silica and incompatible element concentrations, and

lower concentrations of compatible elements, except for anomalous enrichment in Y (Fig. 12).

They cannot be radiometrically dated because of the lack of zircons. However, the presence of

the same QSP alteration as in the other units, and the occurrence of the rhyolite dikes most

commonly within the White Pine area and its surrounding altered rocks shows a temporal and

spatial connection with the White Pine or Red Pine magmas, and therefore a likely genetic

relation.

Most trace elements in the rhyolites follow the differentiation trends defined by the White

Pine and Red Pine phases, except for Nb, Y, and Sc, which are enriched in the rhyolites (Fig. 5,

10). This may be evidence that the rhyolites evolved from a separate magma; however, these

anomalous trends could be explained by the lack of titanite and mafic phases that would

41 fractionate these elements out of the highly-differentiated melt. Therefore, during near eutectic

fractional crystallization, partition coefficients for Nb, Y, and Sc would be low. These elements

were thus concentrated in the residual highly differentiated melt, which was then intruded

upward. It is unlikely that these anomalous trace element enrichments are simply another

manifestation of alteration; such HFSE are generally immobile during hydrothermal alteration.

Consequently, we interpret the rhyolites as being highly differentiated residual liquids from the

White Pine or Red Pine magma. These enrichment trends are not unique to the rhyolites in this

region; anomalous enrichment in Sc also occurs in the Pine Grove rhyolites (Keith, 1982) and

Bishop Tuff rhyolites (Hildreth, 1979).

Construction of the Pluton

The new field, geochronologic, and geochemical data allow us to refine the sequence and time scale over which the composite intrusion formed, intruded, chemically evolved, and became

Mo-mineralized. The Little Cottonwood stock intruded between around 31 to 29 Ma, based on

U-Pb zircon evidence. The intrusion was large--with an average diameter of about 12 km and a thickness of at least 6 km based on depths of emplacement from John (John, 1989), which gives an approximate minimum volume of 680 km3--and therefore must have cooled slowly. While the

Little Cottonwood stock was still partially molten, hot, mafic magma, now represented by the

mafic enclaves, was injected. This may have provided a heat source to keep the magma reservoir

partially molten for an extended period of time. About 2-3 m.y. later, the more silicic White Pine

intruded as a separate magma in the northeast section of the stock. Inherited zircons and

gradational contacts suggest that the Little Cottonwood magma may have still been partially

molten at the time of intrusion. After the White Pine intrusion mostly crystallized, the Red Pine

42 magma intruded it. The presence of ground cover, alteration envelopes, and glacial deposits

make it difficult to find contacts within the pluton, but it is clear that there are several

unconnected dike-like occurrences of porphyritic and phaneritic Red Pine. This supports the idea

of a Mo-W deposit that was formed by repeated intrusions of small batches of water-rich Red

Pine magma.

Such complex multistage, mutually intruding small intrusions are common in other

porphyry Mo systems, including the Henderson, Colorado (Carten et al., 1988), Climax,

Colorado (Audétat, 2015), Pine Grove, Utah (Keith et al., 1993), and Endako, British Columbia

(Villeneuve et al., 2001). At Endako, for example, the deposit is hosted within granitic rocks

with zircon ages ranging from 149-145 Ma. The oldest, outermost biotite-hornblende granodiorite is intruded by the youngest, ore-bearing, altered, biotite monzogranite. This mirrors the pluton construction of the White Pine deposit, with an outer, fresh biotite-hornblende granodiorite being intruded by a biotite granite that provided the mineralizing fluid and molybdenum, also covering a period of about 4 My.

Evidence for Venting

Some Mo deposits are known to have extrusive episodes in addition to the intrusive components (Mutschler et al., 1981; Keith et al., 1993). Good examples include mineralized magmatic systems at Pine Grove, Utah (Keith et al., 1986), Questa, New Mexico (Lipman, 1983;

Gaynor et al., 2019), and three in Colorado: Mt. Emmons (Thomas and Galey, 1982; Sillitoe,

1985), Climax (Audétat, 2015) and Henderson (Carten et al., 1988; Mercer et al., 2015). No volcanic rocks directly linked to the Little Cottonwood stock have yet been identified; the

Keetley Volcanics found in the Wasatch Igneous Belt are too mafic and too old (34.6 to 35.5

Ma) (Smyk et al., 2018) to have vented from the Little Cottonwood stock magma chamber, and

the volcanic rocks on East Traverse Mountain, at 35.25 ± 0.13 Ma to 35.7 ± 0.6 Ma (Biek, 2005) are likewise too old.

However, there are field and textural evidences that venting occurred. Phreatomagmatic

pebble dikes and the hydrothermal breccia pipe are the most obvious, and give definitive

evidence of at least some aspect of venting from a magma chamber. Another line of evidence of

venting is the presence of quench textures and features in the White Pine intrusion, and

especially the Red Pine porphyry (Fig. 7). Elongate apatite and zircon grains, and the overall

porphyritic-aphanitic texture of the Red Pine porphyry, and to a lesser extend the White Pine

intrusion, point to venting. The White Pine and Red Pine are also the only units that have

embayed/resorbed quartz, which can form by decompression (Nekvasil, 1991).

Another example of this process is the Johnson Porphyry of the Tuolumne Intrusive Suite

in the Sierra Nevada batholith, suggested by Bateman and Chappell (1979) to have textures

consistent with eruption which accompanied degassing, depressurization, and quenching. Among

these textures are embayed quartz and a fine-grained groundmass. Like the Red Pine porphyry,

the Johnson Porphyry is porphyritic-aphanitic with resorbed quartz phenocrysts and K-feldspar

megacrysts.

Based on the above evidence, we suggest that a highly fractionated rhyolitic magma

partially crystallized, concentrating volatiles in the remaining melt phase until the exsolved

volatile portion was released, fueling one or more eruptions, as in Stock et al. (2016).

Decompression melting occurred as Red Pine magma ascended in the volcanic vent, which destabilized quartz and caused the embayed texture. Then, degassing of water from the magma during eruption raised the solidus temperature, causing the remaining interstitial melt fraction to crystallize (Bateman and Chappell, 1979).

When this would have happened in relation to the pebble dike emplacement is unclear.

There must have been some venting/degassing sufficient to cause quenching of the Red Pine

prior to some pebble dike emplacement, because multiple pebble dike thin sections (CH-061A,

CH-066) have entrained clasts of Little Cottonwood stock and porphyritic Red Pine, and possibly

White Pine (Fig. 9). Mineralization predated some of the pebble dikes, because certain dike

samples (CH-027, CH-036, and CH-039) contain molybdenite. It is not unusual for there to be

several generations of pebble dikes (Johnson, 2014), and cross-cutting dikes show that is the case

here as well, so these events were likely partially contemporaneous.

The lack of any previously recognized Little Cottonwood stock extrusive equivalent is

not surprising. Being the most recently emplaced of all the Wasatch Igneous Belt plutons, its

deposits would be the highest stratigraphically. Significant erosion following isostatic uplift

along the Wasatch Fault footwall and may well have removed most of the vented portion of the

Little Cottonwood stock, exposing only the older volcanic rocks underneath.

Molybdenum-Tungsten Mineralization

The architecture of the deposit (Figs. 3 and 14) is such that the Little Cottonwood stock has been intruded by the White Pine in a cupola at the apex of the pluton. Small plugs or fingers of Red Pine porphyry subsequently were emplaced into the White Pine intrusion, possibly providing the source of the ore mineralization, which is manifest in the fracture-controlled alteration of the stockwork zone (Sharp, 1958; Krahulec, 2015). A few late-stage pebble dikes and a breccia pipe, also part of this magmatic-hydrothermal system, emanate upward from the

White Pine, and are now mostly either eroded away or covered. Many late-stage rhyolite dikes cut through a wide area in the White Pine intrusion, Little Cottonwood stock, and possibly Red

Pine porphyry as well, oriented northeast-southwest, according to regional stress at the time

(Sharp, 1958; Kowallis et al., 1995). QSP alteration overprints the entire White Pine intrusion, the Red Pine bodies, and the surrounding Little Cottonwood stock, while potassic alteration is centered in small zones of very similar mineralized rock around the breccia pipe and on both flanks of the west ridge more than half a kilometer away. This may also be the same rock previously mined in molybdenite adits 2 km to the north of the breccia pipe (Sharp, 1958;

Crittenden, 1965), which are now filled in and inaccessible. The Mo-bearing rocks all are at approximately the same paleo-elevation. The northern adits are at about 100 m lower elevation than the breccia pipe, which is consistent with Sharp’s (1958) assertion that the White Pine and the stockwork zone dip to the north. The highest elevation known occurrence of mineralized rock is on the west side of the west ridge, which is consistent with the regional eastward dip, giving the mineralized area an overall northeast dip.

Figure 14. Cartoon diagram illustrating the sequence of events in the pluton from 31 Ma to present. (A) Intrusion of the Little Cottonwood Stock ~31-29 Ma. (B) Intrusion of White Pine magma into the Little Cottonwood Stock ~27 Ma. (C) Intrusion of Red Pine magma ~26 Ma. (D) Venting of the Red Pine phase, and high-T hydrothermal alteration, including mineralization, ~26 Ma. (E) Venting of pebble dikes to the paleo-surface, and beginning of low-T hydrothermal alteration, ≤26 Ma. (F) Occurrence of the East Traverse Mountain landslide, ~6.5 Ma. (G) Current post-glacial topography.

Scheelite occurs in this deposit in a widespread area of at least 6 km2, in an elevation

range of about 1 km, mostly in the distal fringe zone, although there is some scheelite in the

stockwork zone. It is deposited on some of the QSP-altered fracture surfaces in the White Pine

and in the surrounding Little Cottonwood stock (Sharp, 1958). Our heavy mineral separates

contained scheelite primarily in the rhyolite dikes.

The presence of tungsten in the form of scheelite (CaWO4) is unusual. W in Mo deposits

is typically present as wolframite ((Fe,Mn)WO4). While scheelite is recovered from Mo skarns in

high-CaO country rocks associated with arc-related porphyry Mo deposits (Taylor et al., 2012),

the scheelite here is found in veins in the host intrusive igneous rock and disseminated in rhyolite

dikes. More common sphalerite was also identified in the heart of the potassically altered breccia

pipe. While it is not highly unusual to have sphalerite in the heart of an ore-related intrusion, it usually forms along with galena and quartz in more distal or peripheral zones (Ludington and

Plumlee, 2009; Taylor et al., 2012).

Originally Sharp (1958) noted some similarities between the White Pine and the Climax

(Colorado) porphyry Mo deposits. Subsequent workers have identified two discrete types of porphyry Mo deposits, and the White Pine deposit seems less like Climax-types and more closely associated with arc-related porphyry Mo deposits, as classified by Taylor et al. (2012).

Among the parameters in which it fails to meet the Climax-type characterization are: 1) the deposit is smaller and lower ore grade than most Climax-type deposits; 2) the deposit lacks widespread gangue fluorite; 3) wrong concentrations of several trace elements (Zr, Sr, Rb, etc.);

4) the host granitoid is I-type, not A-type, according to the Whalen et al. (1987) classification,

and was emplaced in a magmatic arc, not a post-orogenic extension environment.

Climax-type deposits are typically higher grade than arc-related porphyry Mo deposits, at about 0.3-0.5% molybdenite (0.2-0.3% Mo) (Guilbert and Park, 1986), with typically 100-1,000

Mt of ore (Ludington and Plumlee, 2009), while arc-related deposits have 0.03-0.22% Mo, and are typically greater than 50 Mt (Taylor et al., 2012). The White Pine deposit is estimated to hold between 14.5-16 Mt of Mo at 0.1% Mo grade, or 0.16% molybdenite, and 0.02% WO3

(Krahulec, 2015; Smyk, 2015). The grade and tonnage of ore in the White Pine deposit is thus

more characteristic of arc-related porphyry Mo deposits.

The mineralogy and geochemistry of the deposit is also closer to arc-related porphyry

deposits than Climax-type deposits, although in some ways it is transitional. In Climax-type deposits fluorite is a significant gangue mineral (Ludington and Plumlee, 2009), but in arc- related deposits quartz is the dominant gangue mineral, and fluorite is only minor (Guilbert and

Park, 1986; Taylor et al., 2012). Sharp (1958) noted the presence of fluorite in a geographically limited area in and near the Red Pine porphyry on the west ridge, whereas quartz is present in

stockwork veins, ribbon veins, vugs, bull quartz, and silicified granite throughout the White Pine

deposit.

The source plutons of Climax-type deposits generally have >250 ppm Rb, >20 ppm Nb,

>2000 ppm F, <100 ppm Sr, and <120 ppm Zr (Ludington and Plumlee, 2009). Few whole rock

analyses from the unaltered plutonic rocks of the White Pine deposit fit any of these criteria, and

none fit all of the geochemical criteria. Of the moderately to heavily altered and mineralized

plutonic rocks, a few fit some of these criteria. Three of forty-four total samples have >250 ppm

Rb, four have >2000 ppm F, and only one has <100 ppm Sr. The rhyolite dikes do fit most of

these categories, but are unlikely to be the source of the mineralization because they are all

themselves barren of Mo mineralization. Igneous rocks in arc-related Mo deposits, however,

usually have <300 ppm Rb, <30 ppm Nb, and >100 ppm Sr, which describes virtually all of our

samples except for the rhyolites and three highly altered White Pine samples with high Rb.

Finally, the tectonic setting at the time of emplacement and the type of granite don’t

support Climax-type deposits. There is debate as to the age of onset of post-orogenic extension in

the western North American cordillera, and although there may have been some local extension during the emplacement of the granitoids from ~32-26 Ma, most Great Basin extension began in

earnest much later at about 18 Ma, and extension in the study region possibly later than that

(Best and Christiansen, 1991; Best et al., 2009). Kowallis et al. (1990) suggest displacement

along the Wasatch normal fault began at 11 Ma based on apatite and zircon fission track ages,

while Armstrong et al. (2003) proposed 10-12 Ma using the same techniques and apatite (U-

Th)/He ages. This implies that the units of the White Pine deposit were not emplaced in a truly

extensional, post-subduction tectonic setting as is called for in the model for Climax-type

deposits (Ludington and Plumlee, 2009). In addition, Climax-type deposits are typically related

to A-type granites, but the Little Cottonwood stock, White Pine intrusion, and Red Pine porphyry

are I-type intrusions based on the chemical criteria of the Whalen et al. (1987) Ga/Al-Zr

discrimination diagram, and plot as Volcanic Arc Granites on the Pearce et al. (1984) Y-Nb

discrimination diagram (Fig. 5). This is more consistent with the type of intrusions that host arc-

related deposits, which form in subduction zone settings and are hosted by oxidized calc-alkaline

granitoids. Taylor et al. (2012), however, did not preclude the possibility of hybrid deposit types

between Climax-type and arc-related porphyry Mo-type, and that may be the most fitting

characterization of the White Pine deposit, given that it bears a few similarities to Climax-type

deposits as well.

Connection to the East Traverse Mountain Landslide

It has recently been proposed that East Traverse Mountain (Fig. 1), previously mapped as

essentially a horst block oriented perpendicular to the general north-south trend of the Wasatch

Mountains (Biek, 2005), is actually a rootless landslide block (Keith et al., 2017; Perfili et al.,

2017). Among the various lines of evidence in support of this theory are the pebble dikes in the

East Traverse Mountain that bear intrusive clasts, and the pebble dikes in the White Pine

intrusion. The pebble dikes provide not only support for this hypothesis, but also clues for a

possible mechanism for how this landslide event occurred.

Pebble Dike Evidence Supporting the Landslide Hypothesis

Thin sections of the East Traverse Mountain pebble dikes show most intrusive igneous

clasts are similar to the Little Cottonwood stock, White Pine intrusion, and Red Pine porphyry in texture and mineralogy (Figs. 7 and 9). Mineral separates from East Traverse Mountain pebble dikes also contain molybdenite, uranothorite, and scheelite—minerals distinctive of the Mo-W

mineralization in the White Pine area. Finally, the ages of zircons in pebble dike clast CH-036

from the East Traverse Mountains (Fig. 4) form a peak age within the uncertainties for the age of

the Little Cottonwood stock; moreover, it also has a subsidiary peak and a shoulder on the probability spectrum that are the same age as the White Pine intrusion. The matrix of pebble dike

CH-039 also has zircons with a peak age like that of the Little Cottonwood stock.

We propose that the pebble dikes were generated in the Red Pine porphyry, and perhaps also in the earlier White Pine intrusion, sampling already solidified or mostly solidified portions of the Little Cottonwood, White Pine, and Red Pine intrusions, before penetrating the sedimentary and volcanic rocks now on Traverse Mountain, which rested above the Little

51 Cottonwood stock at that time. Following a radical change in the tectonics of the region accompanying the onset of extension and uplift of the Wasatch Range, East Traverse Mountain subsequently broke loose in a catastrophic landslide, and slid rapidly to its current location.

Areas of cataclastic deformation and pseudotachylite in the region (Fig. 1) are interpreted to be shear zones caused by this event (Perfili et al., 2017; Kindred et al., 2018). In this event, the pebble dikes were cut, with the upper part entrained in the landslide block, and the lower portion rooted in the Little Cottonwood stock/White Pine host. The presence of clasts of the three lithologies found in the White Pine deposit, the presence of molybdenite and scheelite, and the presence of QSP alteration in the pebble dike matrix, make it unlikely that there is any other possible source for the Traverse Mountain pebble dikes.

Possible Alteration Contribution to the East Traverse Mountain Landslide

The presence of the Little Cottonwood stock below the paleo-location of the East Traverse

Mountains may have contributed to the slide. The Tertiary volcanic rocks on the East Traverse

Mountains mostly are argillically altered, and pockets of clays in pebble dikes and clays in samples of the Red Pine and White Pine are evidence of a convecting hydrothermal fluid system, which circulated from the cooling intrusion up into the overlying East Traverse Mountains block.

This argillic alteration may have produced or enhanced a zone of weakness. Alteration could have occurred along a preexisting fault, or the interface between the Little Cottonwood stock and the overlying Paleozoic sedimentary strata, particularly if there was an impermeable layer, such as the thick Manning Canyon Shale or Doughnut Formation (Biek, 2005), along which circulating hydrothermal fluids locally concentrated. Evidence for this scenario is found in brecciated quartzite in the Geneva Rock quarry on the west end of East Traverse Mountain, which is visibly altered in some locations. One such quartzite sample (CH-081) has ~5.3% FeOT

52 and ~2.4% Al2O3, which may represent alteration of more pure quartzites in the area (sample

CH-083), which have closer to 96% SiO2, 1.2% Al2O3, and 0.5% FeOT. Another possibility for

the location of the slide surface is a weakness in the altered intrusive rocks themselves,

especially given the coincidence between stockwork fractures and alteration in the White Pine

intrusion. Whichever the case is, the slide surface is no longer visible in most locations, having

been largely been eroded away from top of the autochthonous block; only small zones of

pseudotachylite and cataclasite zones have been preserved, and the base of the East Traverse

Mountains is buried by valley-fill sediment.

The trigger for the slide is unknown, but it could be attributed to an earthquake along the

Wasatch Fault, which is thought to have been active since at least 10-12 Ma, but perhaps as early

as 17 Ma (Kowallis et al., 1990; Bruhn et al., 2005).

Sequence of Events

The presence of both Little Cottonwood stock and Red Pine porphyry clasts in pebble

dike sample CH-061A (Fig. 9), which is silicified but has no significant sericitic or pyritic

alteration, provides some framework for understanding the evolution of this system (Fig. 14).

The Little Cottonwood stock intruded between 31-29 Ma, and the more mafic source magma of

the enclaves mixed with it shortly afterwards. Then, it was intruded by the White Pine and then

Red Pine magmas, a system which didn’t significantly solidify until about 26 Ma. Venting may

have occurred as early as after intrusion of the White Pine magma, but certainly followed

intrusion of the Red Pine magma. This was manifested at least in the form of pebble dikes, if not

volcanic activity as well. Whichever the case, the venting had to be voluminous enough to cause

the quenching (via release of volatiles) of the apical Red Pine magma, preserving the quench

textures visible in hand sample and thin section. Pebble dike emplacement, fueled by continued

circulation of hydrothermal fluids, either commenced or continued after this quenching, injecting

clasts of Little Cottonwood stock, White Pine, and quenched Red Pine porphyry into the

overlying older sedimentary and volcanic section now exposed on East Traverse Mountain.

Alteration possibly predated and postdated emplacement of some pebble dikes, as seen in some

samples with juxtaposed heavily altered and nearly fresh intrusive igneous clasts of rock types

that are usually altered. Because of this, and cross-cutting relationships showing multiple generations of pebble dikes, it is likely that pebble dike injection and QSP alteration processes partially overlapped, allowing altered and unaltered clasts to be included, as well as possible subsequent alteration of some pebble dikes after emplacement at a shallow level.

After injection of pebble dikes, once the system solidified, there was overprinting of propylitic alteration (based on chlorite and epidote seen in many intrusive samples from the region), and then argillic alteration, manifested by the pervasively altered volcanic rocks in the central and eastern portions of Traverse Mountain, as well as pebble dikes with pockets of clay,

and crandallite-jarosite alteration rimming some of the clasts. This alteration in the overlying

thrust sheet of sedimentary rocks contributed to a plane of weakness that later failed

catastrophically.

At least 10 m.y. later, inception of the Wasatch Fault along the western margin of the

Little Cottonwood stock created several kilometers of displacement, providing the steep

elevation gradient needed for the slide to occur, as well as tilting the pluton 20° to the east

(Armstrong et al., 2003). This tilting and subsequent Pleistocene glacial erosion obscured the

original geometry of the intrusion, which would likely have put the White Pine deposit at the

apex of the intrusion prior to tilting. The slide itself is thought to have occurred about at about

6.5 Ma (Keith et al., 2017; Chadburn et al., 2018; Jordan et al., 2018).

CONCLUSIONS

Investigation of the timeline of events in the construction of the White Pine ore deposit shows a complex magmatic and hydrothermal history over greater than 4 million years. The

Little Cottonwood stock was emplaced in a continental magmatic arc setting beginning a little before 30 Ma, as opposed to previous interpretations of it being in a post-orogenic extensional environment. It crystallized titanite, allanite, hornblende, zircon, and apatite which depleted

REEs, especially Y, and gave the resultant pluton an apparent adakitic geochemical signature.

U/Pb ages and whole rock trace compositions show that the White Pine intruded the Little

Cottonwood stock at 27 Ma and was a distinct batch of magma, not a derivative of the Little

Cottonwood stock magma. Based on regional geology, the presence of mafic enclaves, and Sr and Nd isotopic values, both pulses of magma probably had a mantle component, but they extensively assimilated continental crust. The separate Red Pine magma intruded next at 26 Ma, exsolving volatiles as it crystallized. These fluids contributed to the formation of a hydrothermal system, and triggered venting to form pebble dikes and possibly volcanic eruptions, implied by resorption of quartz and quench textures in Red Pine porphyry. The hydrothermal system formed an arc-related porphyry Mo-W deposit, and caused potassic, QSP, and later propylitic and argillic alteration. Highly evolved late-stage rhyolite dikes also intruded at this time as well, possibly fractionated from the White Pine or Red Pine magmas.

This hydrothermal system likely caused a plane of weakness between the composite pluton and the overlying Paleozoic sedimentary strata. About 20 m.y. later, following development of the Wasatch normal fault as part of Great Basin rifting, the East Traverse

Mountains landslide occurred, displacing down to the west into the graben valley. Key to this

interpretation, pebble dikes in East Traverse Mountain bear clasts resembling Little Cottonwood stock, White Pine, and Red Pine, in mineralogy, texture, age, and alteration mineral assemblage.

These features suggest that those pebble dikes are the upper part of the same pebble dike system found in the White Pine deposit 17 km away, supporting other evidence that the East Traverse

Mountains once rested atop the White Pine porphyry Mo system.

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64 APPENDICES Table A1. Modal mineral data (attached)

Table A2. XRF whole rock data (attached)