GEOSPHERE Mesozoic Magmatism and Timing of Epigenetic Pb-Zn-Ag Mineralization in the Western Fortymile Mining District, East-Central Alaska: Zircon

Research Paper

GEOSPHERE Mesozoic magmatism and timing of epigenetic Pb-Zn-Ag mineralization in the western Fortymile mining district, east-central Alaska: Zircon

GEOSPHERE; v. 11, no. 3 U-Pb geochronology, whole-rock geochemistry, and Pb isotopes

DOI:10.1130/GES01092.1 Cynthia Dusel-Bacon1, John N. Aleinikoff2, Warren C. Day3, and James K. Mortensen4 1U.S. Geological Survey, 346 Middlefield Road, MS 901, Menlo Park, California 94025, USA 13 figures; 5 tables; 5 supplemental files 2U.S. Geological Survey, Denver Federal Center, MS 963, Denver, Colorado 80225, USA 3U.S. Geological Survey, Denver Federal Center, MS 911, Denver, Colorado 80225, USA 4University of British Columbia, 2020-2207 Main Mall, Vancouver, British Columbia, V6T 1Z4, Canada CORRESPONDENCE: [email protected]

CITATION: Dusel-Bacon, C., Aleinikoff, J.N., Day, ABSTRACT on the northeast-trending faults to be a far-field effect of dextral translation W.C., and Mortensen, J.K., 2015, Mesozoic magmatism and timing of epigenetic Pb-Zn-Ag mineralization along Late Cretaceous plate-scale boundaries and faults that were roughly in the western Fortymile mining district, east-central The Mesozoic magmatic history of the North American margin records parallel to the subsequently developed Denali and Tintina fault systems, which Alaska: Zircon U-Pb geochronology, whole-rock geo- the evolution from a more segmented assemblage of parautochthonous and currently bound the region. chemistry, and Pb isotopes: Geosphere, v. 11, no. 3, p. 786–822, doi:10.1130/GES01092.1. allochthonous terranes to the more cohesive northern Cordilleran orogenic belt. We characterize the setting of magmatism, tectonism, and epigenetic Received 11 June 2014 mineralization in the western Fortymile mining district, east-central Alaska, INTRODUCTION Revision received 25 November 2014 where parautochthonous and allochthonous Paleozoic tectonic assemblages Accepted 28 April 2015 are juxtaposed, using sensitive high-resolution ion microprobe (SHRIMP) U-Pb The Fortymile mining district of east-central Alaska (Cobb, 1973) is located Published online 13 May 2015 zircon geochronology, whole-rock geochemistry, and feldspar Pb isotopes of near the juxtaposition of two Paleozoic tectonic assemblages that developed Mesozoic intrusions and spatially associated mineral prospects. New SHRIMP along the northwestern margin of Laurentia, one parautochthonous and the U-Pb zircon ages and published U-Pb and 40Ar/39Ar ages indicate four episodes other allochthonous (Fig. 1). The middle to late Paleozoic evolution of these of plutonism in the western Fortymile district: Late Triassic (216–208 Ma), Early and other related pericratonic assemblages of the northern Cordillera has been Jurassic (199–181 Ma), mid-Cretaceous (112–94 Ma), and Late Cretaceous well documented (e.g., Dusel-Bacon et al., 2006; Nelson et al., 2006; Piercey et (70–66 Ma). All age groups have calc-alkalic arc compositions that became al., 2006). However, fewer studies have addressed the Mesozoic amalgama- more evolved through time. Pb isotope compositions of feldspars from Late tion to postamalgamation magmatic history of east-central Alaska (Newberry Triassic, Early Jurassic, and Late Cretaceous igneous rocks similarly became et al., 1998a; Hansen and Dusel-Bacon, 1998; Dusel-Bacon et al., 2002, 2009; more radiogenic with time and are consistent with the magmas being man- Allan et al., 2013). Mesozoic magmatism records the interaction of outboard tle derived but extensively contaminated by upper crustal components with allochthonous terranes with the inboard amalgamated continental margin and evolving Pb isotopic compositions. Feldspar Pb isotopes from mid-Cretaceous thus helps constrain plate tectonic models of the northern Cordillera. Mesozoic rocks have isotopic ratios that indicate magma derivation from upper crustal intrusive rocks in the Fortymile mining district are also important because of sources, probably thickened mid-Paleozoic basement. The origin of the mantle their spatial association with undated epigenetic mineral prospects. The dis- component in Late Cretaceous granitoids suggested by Pb isotopic ratios is trict is known for its placer gold (Yeend, 1996), and includes dozens of epigen- uncertain, but we propose that it reflects asthenospheric upwelling following etic base and precious metal prospects (Werdon et al., 2004a; Dusel-Bacon et slab breakoff and sinking of an inactive inner subduction zone that delivered al., 2003, 2009; Siron et al., 2010; Allan et al., 2013), especially in the western the previously accreted Wrangellia composite terrane to the North American part of the district in the southwestern Eagle 1° × 3° quadrangle (Figs. 2 and 3). continental margin, after the outer Farallon subduction zone was established. Our study documents magmatic events, structures, and regional tectonics Epigenetic Pb-Zn-Ag ± Cu prospects in the western Fortymile district are that are important to understanding the metallogeny and mineral potential of this spatially associated with splays of the northeast-trending Kechumstuk sinis- economically important time frame in the Fortymile district, in adjacent Yukon tral-normal fault zone and with ca. 68–66 Ma felsic intrusions and dikes. The (Allan et al., 2013), and throughout much of Alaska (Goldfarb, 1997). This paper similarity between Pb isotope compositions of feldspars from the Late Creta- complements a new geological map of the Mount Veta area (by Day et al., 2014), ceous igneous bodies and sulfides from the epithermal prospects suggests a guided by airborne geophysical surveys (Burns et al., 2008), that provides a struc- For permission to copy, contact Copyright Late Cretaceous age for most of the mineralization. Fluid flow along the faults tural framework for understanding the metallogeny and tectonic evolution of the Permissions, GSA, or [email protected]. undoubtedly played a major role in mineralization. We interpret displacement study area. We present petrologic, whole-rock major, minor, and trace element

GEOSPHERE | Volume 11 | Number 3 Dusel-Bacon et al. | Mesozoic magmatism and mineralization, Fortymile district, Alaska Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/3/786/3336037/786.pdf 786 by guest on 27 September 2021 Research Paper

Ko 68 tzeb o W N o Soundue Beaufort AK

140 Fortymile Sea mining SD district o W Y.T.

160

Area of

eastern B.C. Kaltag fault map

67oN

Y.T.

Fb o limit W pYTa of E Fig. 3 YT

D ordill

W Tintina fault o T Fig.. 2 SEL eran 148 Denali fault YT WYN BASIN Oceanic assemblage Paleozoic - Jurassic Tozitna - Angayucham

terranes YT deformation Devonian - Triassic Finlayson Slide Mountain - Seventymile AK Lake terranes Y.T. YT Arc assemblages Wh YT Devonian - Permian Stikine - Boswell assemblage WL Y.T. 60o N Permian 59o Klondike assemblage N B.C. Pennsylvanian - Permian Pacific nana terrane Klinkit assemblage Devonian - Mississippian Ocean kon- Ta Fortymile River, Snowcap and

Yu Finlayso n assemblages Continent margin assemblages YT Devonian - Mississippian Alaska Range and pY Ta Yukon-Tanana Upland Neoproterozoic - Devonian North American basinal facies Neoproterozoic - Devonian 56oN North American

platformal facies 128

Scale AK Neoproterozoic - Paleozoic 0200 B.C.

Other continent margin W assemblages km

Figure 1. Paleozoic tectonic assemblages of the northern Cordillera (modified from Dusel-Bacon et al., 2006; Nelson et al., 2013). Map abbreviations for lithotectonic terranes and assemblages: pYTa—parautochthonous Yukon-Tanana assemblage; YT—allochthonous Yukon-Tanana terrane. Place abbreviations: AK—Alaska; B.C.— British Columbia; D—Dawson; E—Eagle; Fb—Fairbanks; NWT—Northwest Territories; Wh—Whitehorse; WL—Watson Lake; T—Tok; YT—Yukon. Unpatterned areas within the Cordillera are undivided accreted terranes. Red rectangle shows the location of study area. Location of the Fortymile mining district is shown in inset.

ARC AND BASINAL ASSEMBLAGES 148 30′ 147 00′ 141 00′ (ALLOCHTHONOUS YUKON-TANANA TERRANE) 66 00′ 144 00′ Klondike Schist Permian felsic metaigneous rocks; minor metasedimentary and mafic metaigeous rocks Nasina assemblage Qtz-Ser-Chl schist, marble; carbonaceous rocks; Mississippian and Permian felsic metaigneous rocks OCEANIC & HIGH-P ASSEMBLAGES Chicken metamorphic complex Intermediate-mafic metaigneous Seventymile terrane rocks, quartzite, Mississippian(?) marble Peridotite, greenstone; Mississippian to Fortymile River assemblage Bt schist and gneiss, marble, Triassic chert and limestone quartzite; Mississippian metaigneous gneiss (brown) Chatinika assemblage (near Fairbanks) Ladue River unit Qtz-Chl-Ser schist and mafic and felsic Eclogite, marble, schist metaigneous rocks (in part Devonian-Mississippian) Wickersham grit unit iver R Yukon R Figure 2. Generalized geologic map of ive CONTINENTAL MARGIN ASSEMBLAGES Birch Creek r (PARAUTOCHTHONOUS YUKON-TANANA ASSEMBLAGE) east-central Alaska and adjacent part of a

A Yukon showing Mesozoic and Cenozoic nik Tintina faul Butte assemblage, Alaska Range equivalents ata Ch Devonian-Mississippian metavolcanic rocks, phyllite, marble granitoids, Paleozoic arc, basinal, and

Blackshell and Dan Creek units; Keevy Peak Fm. Carbonaceous continental margin assemblages, and YUKON Livengood ALASK metasedimentary rocks, marble, Devonian-Mississippian metarhyolite 65 00′ Circle Charley River t oceanic and high-pressure (P) rocks. Lake George and Fairbanks-Chena assemblages Qtz-mica schist, Geology in Alaska is from Foster (1992) Fort Knox amphibolite, quartzite; Devonian-Mississippian augen gneiss and Dusel-Bacon et al. (1993, 2002, (dark blue) Ch er 2006), and in Yukon is from Mortensen ena iv Eagle PLUTONIC AND VOLCANIC ROCKS R (1988, 1996) and Gordey and Ryan Fairbanks (2005). Northeast-trending fault (from Tg Tv Ts&QTb T Wilson et al., 1985; Foster et al., 1994; a n a O’Neill et al., 2010; Day et al., 2014) ab- n B a Salcha River SCF lKglKv mKg mKv Jg Trg breviations: BMTZ—Black Mountain tectonic zone; KF—Kechumstuk fault; W lt oo u Pogo mile d fa rty R SMF—Sixtymile fault; SCF—Shaw k o iver R R e Fig. 3 F iv i re Creek fault. Mineral abbreviations in e ve C r r aw Sh unit descriptions: Bt—Biotite; Chl— Chicken Chlorite; Qtz—Quartz; Ser—Sericitic

South Fk Big Delta South Fk Fairbanks Taylor n muscovite. Age and rock type abbrevi- Big DeltaDelta BMTZ Mountain Dawso 64 00′ EagleMHF Dawson 138 00′ ations in Plutonic and Volcanic Rocks: KF 3 MoMosquitsquito 212 ± 3 Ma Q—Quaternary; T—Tertiary; lK—Late r Y e u Cretaceous; mK—mid-Cretaceous; J— v u i o k R D Fork o Jurassic; Tr—Triassic; b—basalt; g— a n

t C n l

e granite; v—volcanic rocks; s—sedi

D 24, 25

k N r

ines Creek Str o a R H nd of D E R mentary rocks. Unpatterned areas r enali F Fa 67 ± 1 Ma t ult h ii v

L e are Neoproterozoic to Cambrian and A n a F iso d r Den u Devonian sedimentary rocks north McKinley e

Stran d R of Dena SM

li F i

aul iver v of the Tintina fault, and Quaternary t e e R

u r r d ve surficial deposits elsewhere. Sensi- ana R a t Ri Tan ive L war r Ste tive high-resolution ion microprobe iver Tok hite R (SHRIMP) zircon U-Pb ages and un- Den W 0 30 ali certainties (rounded to nearest mil- fau lion years) and corresponding map Healy lt Mt. Hayes Stewart River numbers (Table 2) are shown for three 63 00′ KM Tanacross of our dated samples collected in the Tanacross quadrangle that are outside Contact—dashed where concealed or uncertain our study area (outlined in red and High-angle fault—Arrows show direction of strike-slip movement; shown in Fig. 3). dashed where inferred; dotted where concealed Major town High-angle fault inferred from K-Ar age domains and (or) 3 magnetic data; dotted where concealed (Wilson et al., 1985; SHRIMP U-Pb zircon sample number and age from 212± 3 Ma Sanchez et al., 2013) outside of study area (Fig. 4) A Prospects discussed in text: (A) Peternie; (B) Lead Creek and Thrust fault—Sawteeth on upper plate; dashed where inferred Champion Creek; (C) Mosquito; (D) Pika Canyon; (E) Bluff and Taurus Low-angle normal fault—Sawteeth on upper plate; dotted where (Werdon et al., 2004) concealed Lode gold mine discussed in text Fort Knox Quadrangle boundary—Quadrangle names in lower left-hand corner

64°25′

Eisenmenger Fork

Middle Fork 20 caldera 70±1 19 70±1 Mount Harper 71±1 Fish 30 Figure 3. Generalized geologic map 66±2 187±3 of the western Fortymile district 64°15′ 29 14 S 210±3 showing the map numbers and sen- A Fig. 4 Oscar WLWM 103±2 26 31 sitive high-resolution ion microprobe 188±2 6 B C 66±1 (SHRIMP) U-Pb zircon ages of igneous 16 96±1 Fork 11 68±1 27 LWM samples analyzed in this study and epi- 22 10 Section 21 MountMount 181±3 genetic base and precious metal pros- 15 Veta pects discussed in text. Geology and Mount Harper Fault 5 184±4 Little 101±1 191±5 faults are modified from Foster (1976, Enchilada 1992) and Day et al. (2014). SHRIMP Drumstick U-Pb zircon ages and uncertainties

Molly

are rounded to nearest million years. r K Diamond e ech Abbreviations: LWM—Little Whiteman; v hu Mountain i m WLWM—West Little Whiteman; as- R Middle s ly tu Hea Cr k sem.—assemblage; met.—metamor- 216±4 Kechumstuk faultFault zone Zone 23 phic. Mineral abbreviations in unit de- eek 208±3 Eva 1 68±1 scriptions: Bt—Biotite; Chl—Chlorite; 4 Mitchell Qtz—Quartz; Ms—Muscovite. Lettered 185±3 8 7 21 68±1 mineral prospects in Mount Harper 186±3 2 215±4 area: A—generalized location for Larsen 185±3 Cr Ridge prospect, Lucky 13 prospect, and eek unnamed prospect near the vertical 9 18 angle benchmark “Good”; B—Airplane 94±1 Ridge prospect; C—quartz-porphyry 0 5 10 MILES 17 96±2 109±2 prospect (see Werdon et al., 2004a, and 13 Kechumstuk references therein). Prospects are de- 12 28 Mountain 0510 15 KILOMETERS 112±2 ≤~65 scribed in text (or see Table 1). 64°00′ 144°00′ 143°30′ 143°00′ 142°30′ Quaternary Mid-Cretaceous Mid-Paleozoic Alluvium and colluvium Granite and granodiorite Qtz-Chl-Ms schist, quartzite, marble, greenstone (Nasina assem.)

Tertiary Early Jurassic Greenstone, phyllite, marble, quartzite (Chicken met. complex) Intermediate volcanic rocks Granodiorite, monzonite, Bt schist, quartzite, marble, amphibolite (Fortymile River assem.) and granite Late Cretaceous Late Triassic Augen gneiss and orthogneiss (Lake George assem.) Felsic volcanic rocks Granodiorite, tonalite, Felsic gneiss and granitoids, undivided (Lake George assem.) Granite and quartz monzodiorite 23 High-angle fault—Arrows show direction of strike-slip movement. 68±1 SHRIMP U-Pb zircon sample number and age (Ma) Ball and bar on downthrown block. Dashed where inferred. Eva Mineral prospect discussed in text Thrust fault—Sawteeth on upper plate. Dotted where concealed

geochemical data, and zircon U-Pb ages for felsic and intermediate-composition west-dipping subduction (present-day coordinates) beneath the rifted crustal Mesozoic igneous rocks in an ~2500 km2 area in the western Fortymile mining fragment as recorded in Permian felsic arc volcanic rocks (Klondike assem- district and comparable data for a Late Cretaceous alkalic pluton in the eastern blage; Fig. 1) (Tempelman-Kluit, 1979; Mortensen, 1992; Erdmer et al., 1998; part of the district. We also report new Pb isotopic compositions of feldspar from Creaser et al., 1997; Hansen and Dusel-Bacon, 1998; Nelson et al., 2006). On Mesozoic igneous rocks in the study area together with previously published the basis of detrital zircon data, it was proposed (Beranek and Mortensen, 2011) analyses from east-central Alaska to (1) characterize the geochemical evolution that the YTT was in proximity to or had overridden the Laurentian margin of of Mesozoic magmatism, and (2) compare the feldspar Pb compositions with northwestern Canada and, hence, that the Slide Mountain ocean was narrow both new and previously published sulfide Pb compositions from prospects in or had closed by Early to Middle Triassic time. the study area to identify the probable causative magmatic episodes for the epi- In Alaska, the allochthonous YTT was affected by pre–Late Triassic and Early genetic mineralization. Using these data sets, we evaluate and revise previous Jurassic deformation and metamorphism (Hansen and Dusel-Bacon, 1998; Day models for the Mesozoic tectonic and magmatic history and metallogenesis of et al., 2000; Berman et al., 2007; Beranek and Mortensen, 2011) and intrusion the Fortymile district (Newberry et al., 1998a; Hansen and Dusel-Bacon, 1998; Du- of Late Triassic and Early Jurassic granitoids (Fig. 2). Following Early Jurassic sel-Bacon et al., 2002, 2009; O’Neill et al., 2010) and the northern Cordillera (Allan contraction, the combined Alaskan continental margin, composed of the upper et al., 2013). Our integrated new explanation for the study area can be tested in plate allochthonous YTT and lower plate parautochthonous YTa, was affected by other parts of the Yukon-Tanana Upland that have been mapped in less detail and the following events: (1) Middle Jurassic to Late Cretaceous north-dipping sub- elsewhere in the northern Cordillera. The hydrothermal aspect of the model, spe- duction and collision of the Wrangellia composite terrane with the Alaskan con- cifically the Late Cretaceous phase of mineralization suggested by sulfide and tinental margin (Nokleberg et al., 1985; Plafker and Berg, 1994; Trop et al., 2002); feldspar Pb isotope data from the Fortymile area, can be tested in other parts of (2) mid-Cretaceous extension followed by intrusion of postkinematic granitoids the northern Cordillera with direct dating of minerals associated with hydrother- (Wilson et al., 1985; Newberry et al., 1998b) and formation of rhyolitic calderas mal mineralization or with comparable Pb isotope data for sulfides from mineral (Bacon et al., 1990; Mortensen and Dusel-Bacon, 2014) (Fig. 2); and (3) Late Cre- prospects and feldspars from associated intrusions. taceous to Paleocene subduction, associated granitic plutonism and volcanism (Wilson et al., 1985; Newberry et al., 1998a; Bacon and Lanphere, 1996; Bacon et al., 2014), and dextral-oblique compression of the continental margin and north- REGIONAL GEOLOGIC AND TECTONIC FRAMEWORK westward movement between the right-lateral Tintina and Denali fault systems (Fig. 2) and preexisting parallel faults (Plafker and Berg, 1994). The two Paleozoic pericratonic assemblages juxtaposed in the Fortymile Geologic evidence from western Canada and eastern Alaska suggests mining district are (1) a parautochthonous continental-margin assemblage ~430 km of mostly Eocene dextral displacement across the Tintina fault sys- that contains metamorphosed Devonian–Mississippian rocks, the protoliths of tem (Gabrielse et al., 2006). Most of the ~370 km dextral displacement on the which were granitoids and bimodal within-plate volcanic rocks (Fig. 1), and (2) Denali fault system is thought to be post–Early Cretaceous, much of the move- an allochthonous assemblage that was rifted from the continental margin in ment having occurred in mid-Tertiary time (Dodds, 1992; Lowey, 1998). North- Early Mississippian time and contains Mississippian arc- and backarc-related east-trending faults, with both left-lateral and vertical displacements (Wilson rocks (Fig. 1). To the east in Canada, Pennsylvanian and Permian arc-related et al., 1985; Foster et al., 1994; Dusel-Bacon and Murphy, 2001; Dusel-Bacon metamorphosed intrusive and volcanic rocks (Fig. 1) intrude and overlie et al., 2009; O’Neill et al., 2010), are mapped or inferred, based on geophysical the allochthonous continental margin assemblage (e.g., Nelson et al., 2006; data, throughout the Yukon-Tanana Upland (Fig. 2). Movement along the north- Piercey et al., 2006). Following the terminology in Dusel-Bacon and Williams east-trending faults has been attributed to the clockwise rotation of blocks re- (2009), the continental-margin component, interpreted to have remained sulting from the right-lateral movement along the Tintina and Denali faults inboard of the intervening ocean, is referred to as the parautochthonous (e.g., Page et al., 1995; O’Neill et al., 2010). Yukon-Tanana assemblage (YTa), whereas the rifted component is referred to as the allochthonous Yukon-Tanana terrane (YTT) (Fig. 2). Crustal extension and rifting of the continental margin resulted in formation of a long-lived (Late GEOLOGIC SETTING OF THE WESTERN FORTYMILE DISTRICT Devonian to Early Triassic) ocean basin between the allochthonous YTT and the parautochthonous YTa (e.g., Tempelman-Kluit, 1979; Hansen, 1990; Nelson Geology et al., 2002, 2006; Dusel-Bacon et al., 2006). Remnants of this ocean basin, preserved primarily as discontinuous klippen or fault slivers, make up the Slide Mesozoic magmatic rocks and associated epigenetic Pb-Zn-Ag ± Cu pros- Mountain–Seventymile terrane (Fig. 1). pects in the eastern part of the Yukon-Tanana Upland primarily occur within Subsequent closure of the intervening ocean basin and northward trans- components of the allochthonous YTT, including the Fortymile River and Na- port of the allochthonous YTT was caused by dextral translation and south- sina assemblages, and the Chicken metamorphic complex of Werdon et al.

(2001) (Figs. 2 and 3) (Dusel-Bacon et al., 2006, 2009, 2013). The Fortymile River Ma; U-Pb systematics in coexisting zircon are complicated by Paleoproterozoic assemblage comprises amphibolite facies metasedimentary and metavolcanic inheritance, but suggest a time of igneous crystallization consistent with the ti- rocks, marble, and granodioritic to tonalitic orthogneiss of Early Mississippian tanite age (Dusel-Bacon et al., 2009). A small hornblende quartz monzonite plu- age. The Nasina assemblage consists of (1) a sequence of greenschist facies ton east of the Mount Veta intrusion and adjacent to the Fish prospect yielded a carbonaceous quartzite, phyllite, and schist, (2) marble and greenstone, and (3) SHRIMP U-Pb zircon age of 187 ± 3 Ma (29 in Fig. 3) (Dusel-Bacon et al., 2009). minor felsic volcanic rocks of both Mississippian and Permian age. The Chicken In the western part of the study area, the mid-Cretaceous (ca. 112–105 Ma, metamorphic complex of Mississippian age consists of greenschist facies me- i.e., late-Early Cretaceous) Mount Harper batholith (Newberry et al., 1998a; Du- tavolcanic rocks and subordinate metagabbro, metadiabase, marble, slate, sel-Bacon et al., 2002; Day et al., 2007) intruded Devonian augen gneiss and quartz-mica phyllite, and minor quartzite that occur along the eastern and west- felsic gneiss of the Lake George assemblage (part of the parautochthonous ern margins of the Taylor Mountain batholith (Figs. 2 and 3). Similarities in pro- YTa) and schist and quartzite of the Nasina and Fortymile River assemblages toliths, arc geochemical signatures of metaigneous rocks, and conodont age of the YTT (Fig. 3). In the northern part of the study area, the ca. 70 Ma Middle ranges of rocks of the Chicken metamorphic complex and the Fortymile River Fork caldera (Fig. 3) forms a 10 × 20 km area of rhyolitic welded tuff and granite assemblage suggest a shared origin for both units (Dusel-Bacon et al., 2006, porphyry (Bacon and Lanphere, 1996; Bacon et al., 2014). Altered rhyolite por- 2013), the difference likely being one of metamorphic grade. Mylonitic tonalitic phyry collected from drill core at the Fish mineral prospect (Table 1), located gneiss interlayered with mafic metavolcanic rocks of the Chicken metamorphic ~12 km east of the Middle Fork caldera, yielded a SHRIMP U-Pb zircon age of complex yielded a sensitive high-resolution ion microprobe (SHRIMP) U-Pb 70.5 ± 1.1 Ma (30 in Fig. 3) (Dusel-Bacon et al., 2009). zircon protolith crystallization age of 332.6 ± 5.6 Ma (Dusel-Bacon et al., 2013). The three Paleozoic components of the allochthonous YTT were intruded by Late Triassic, Early Jurassic, mid-Cretaceous (herein, igneous crystalliza- Faults tion ages between ca. 112 and 94 Ma), and Late Cretaceous granitoids (Figs. 3 and 4) (Dusel-Bacon et al., 2009; Day et al., 2014). Late Triassic intrusions form Northeast-trending faults and their kinematically related northwest-trend- large batholiths including the ca. 215 Ma pluton of Kechumstuk Mountain in ing faults are major structural features in the central part of the study area our study area (Fig. 3), the ca. 214 Ma Happy granite of Newberry et al. (1998a), (Burns et al., 2008; Day et al., 2014) (Figs. 3 and 4). The dominant feature is which crops out in the northeastern part of the study area and continues to the the Kechumstuk fault zone, which occurs in a prominent northeast-trending northeast into the central Eagle quadrangle, and the ca. 212 Ma Taylor Moun- topographic depression east of the ridge formed by the Mount Veta intrusion tain batholith in the southeastern Eagle quadrangle (Aleinikoff et al., 1981; (Mount Veta ridge) (Fig. 4). Another northeast-trending fault zone is to the west Cushing, 1984; Werdon et al., 2001) (Fig. 2). The pluton of Diamond Mountain of the Mount Veta ridge. Northwest- and west-trending faults locally crosscut crops out between the pluton of Kechumstuk Mountain and the Taylor Moun- the northeast-trending faults. The mapped faults are zones of brittle damage tain batholith and yields a hornblende 40Ar/39Ar plateau age of 197.3 ± 0.7 (New- that record recurrent fault movement, as evidenced by superimposed offset of berry et al., 1998a) and a U-Pb titanite age of 201.0 ± 1.4 Ma (Dusel-Bacon et preexisting brittle deformation fabrics along fault surfaces, rebrecciated silicic al., 2009) that, considered together, indicate a crystallization age of ca. 199 Ma, and iron-oxide alteration that was focused within the faults, and brecciation straddling the boundary between the Late Triassic and Early Jurassic Periods. of younger granitic and rhyolitic dikes and small intrusions that were locally A prominent northeast-trending ridge is formed by the ca. 188–180 Ma Early emplaced in the fault zones. The northeast-trending faults have much wider Jurassic intrusion that crops out primarily south of Mount Veta (Cushing, 1984; damage zones (as wide as 200 m) and longer trace lengths than the narrower Dusel-Bacon et al., 2002, 2009; Day et al., 2014) (Figs. 3 and 4), herein referred (as wide as 10 m) northwest- and west-trending faults. to as the Mount Veta intrusion. The intrusion was originally designated the sy- The Mount Harper batholith is cut by the steep, northeast-trending, Mount enite of Mount Veta by Foster (1976), but we found no true syenite. Instead, Harper fault that accommodated uplift of the Mount Harper block (Newberry much of the Mount Veta intrusion is composed of coarse-grained, K-feldspar et al., 1998a; Dusel-Bacon and Murphy, 2001; Dusel-Bacon et al., 2002; Bacon megacrystic, monzonite, quartz monzonite, or quartz monzodiorite. The Mount et al., 2014). The contact between the Lake George and Fortymile River assem- Veta intrusion and several small apophyses form a northeast-striking elongate blages is not exposed but is interpreted to be a fault based on the tectonic body that appears to be a thick, east-dipping sill, the base of which is character- relationship of these assemblages in the eastern part of the Eagle quadrangle ized by a migmatitic zone of Fortymile River assemblage country rock invaded (Hansen and Dusel-Bacon, 1998) and on the occurrence of a northwest-trend- by the Mount Veta intrusion (Day et al., 2014). Ductile tectonic foliation is de- ing high-angle fault that we infer continues along Molly Creek and coincides veloped near footwall and roof pendant zones of the Mount Veta intrusion and with the contact between these two assemblages (Figs. 3 and 4). The occur- is parallel to that of the dominant regional penetrative fabrics in the country rence of a fundamental structural contact between the parautochthonous and rock. Titanite from quartz monzonite from the Mount Veta intrusion yielded a allochthonous packages of rocks is suggested by an ~170 × 400 m tectonic lens concordant thermal ionization mass spectrometry (TIMS) age of 185.4 ± 1.2 of mantle-derived, Alpine-type metaharzburgite that is exposed within a panel

64°15’ k C ow G B 65.8 o r l e d v l k i LWM ± 1.5 Ma B West S C o n t t LWM 181.2 a o m Oscar m e t ± 2.6 Mai C h k 67.9 65.8 W Quaternary te it ± 1.1 Ma ± 1.4 Ma L 332.6 Alluvium and colluvium Little T ± 5.7 Ma el Late Cretaceous Enchilada eg ra V ph Rhyolite dike x eta C C k 190.5 k Granite ± 4.8 Ma Mt. Veta 183.4 ± 3.6 Ma Mid-Cretaceous Granite and leucogranite Quartz-feldspar porphyry dike and small intrusion 64°10’ Early Jurassic Granodiorite, monzonite, and granite Late Triassic Granodiorite, tonalite, and ll Ck K Norve ec quartz monzodiorite hum st uk C k Mid-Paleozoic Drumstick Quartz-mica schist, quartzite, marble, and greenstone (Nasina assemblage) C 215.5 ar y Ck 207.9ibo Kechumstuk fault zone M ± 3.4 Ma u C Greenstone, phyllite, marble, and quartzite (Chicken metamorphic complex) ± 2.9 Ma k Eva Kechu E 184.8 mstuk C k Biotite schist, quartzite, marble, and amphibolite (Fortymile River assemblage) va ± 2.9 Ma Ck Augen gneiss and orthogneiss (Lake George assemblage) 68.1 ± 0.8 Ma Our Ck 186.2 Mitchell Shear zone-intense brittle deformation damage zone ± 3.0 Ma 64°05’ Kechumstuk fault zone I ro n C 215.0 k 184.5 Low angle fault-dotted where concealed ± 3.5 Ma ± 3.0 Ma 93.9 ± 1.3 Ma High angle fault-dashed where inferred, dotted where concealed 215.0 SHRIMP U-Pb zircon age (dot is sample location) 95.8 k ± 3.5 Ma r C ± 1.5 Ma Ceda 108.8 Mineral prospect Ck ± 1.7 Ma er pp Mitchell Co

k C y l l o M 0 1 2 MILES < 65 Ma 111.8 ± 1.5 Ma 0 1 2 3 KILOMETERS 64°00’ 143°20’ 143°10’ 143°00’

Figure 4. Bedrock geologic map of the Kechumstuk fault zone area in the western Fortymile mining district showing new SHRIMP (sensitive high-resolution ion microprobe) zircon U-Pb ages and mineral prospects. Geology and faults are modified from Day et al. (2014).

TABLE 1. MAJOR BASE AND PRECIOUS METAL PROSPECTS IN THE WESTERN FORTYMILE MINING DISTRICT, ALASKA Prospect name* Metals Mode of occurrenceComments and references Larsen Ridge, Lucky 13, and W-Cu-Mo ± Ag ± Pb ± Zn W skarn or Skarns are developed in Paleozoic amphibolite facies biotite gneiss, amphibolite, quartzite, unnamed prospect near Mo-W stockwork and minor marble of the Lake George assemblage and are spatially associated with W- vertical angle benchmark and Mo-bearing, primarily quartz monzonite, mid-Cretaceous intrusions (Werdon et al., “Good” (A) 2004a, and references therein). Airplane Ridge (B) Mo-W ± Cu ± Ag ± Au ± Sn porphyry Werdon et al. (2004a, and references therein) Quartz-Porphyry (C) Mo-W ± Cu ± Ag ± Au ± Sn porphyry Werdon et al. (2004a, and references therein) Section 21 Mo-W ± Cu ± Ag ± Au ± Sn porphyry Mineralization is associated with a quartz porphyry-aplite plug that contains quartz-molybdenite veining. Most quartz veins are < 1 mm wide and some contain W ore minerals (Werdon et al., 2004a, and references therein). Little Whiteman (LWM) Pb-Zn-Ag ± Cu carbonate replacement Sulfide replacement occurs in hydrothermally dolomitized carbonate rocks in the hanging and manto wall of the Kechumstuk fault. Smaller sulfide bodies occur high in the LWM section along sympathetic faults distal to the main structure, as stratabound manto bodies and as fracture-fill carbonate-sulfide veins (Siron et al., 2010). Mitchell Cu-Zn-Pb-Au-Ag Au-rich Cu skarn Consists of Cu-, Zn-, and Pb-sulfides and Au- and Ag-bearing minerals in calc-silicate skarn and silicified marble in a roof pendant of Paleozoic quartz-mica schist and greenstone within a Late Triassic granodiorite pluton (Werdon et al., 2004a; Dusel-Bacon et al., 2009). Little Enchilada Mo ± Cu ± Pb ± Ag ± Au quartz-molybde- Quartz-molybdenite veins are in hydrothermally altered granite between two strands of north- nite veins east-striking faults of the Kechumstuk fault zone (Werdon et al., 2004a; Day et al., 2014). West Little Whiteman Zn-Pb-Cu skarn Consists of small pods and veins of dominantly clinopyroxene with sphalerite and lesser galena. Drilling indicated that the marble is underlain by and interbedded with mafic me- tavolcanics (N. King, 2012, written commun.). The marble is part of the same sequence, but at a slightly higher metamorphic grade, as that that hosts mineralization at LWM. Fish Zn-Pb-Cu ± Ag ± Au oxidized skarn Prospect is a deeply oxidized (gossan to 23 m thick) massive sulfide body that is compositionally zoned with Zn-, Ag-, Au-, Pb-, and Cu-rich sections; mineralization is also strongly anomalous in Bi, As, and Sb (J. Light, 2008, written commun.). Occurs adjacent to pluton that yielded 187 ± 3 Ma U-Pb zircon age (29 in Fig. 3) (Dusel-Bacon et al., 2009). Eva Creek Ag-Zn-Pb-Cu skarn?; carbonate Consists of argentiferous galena, sphalerite, chalcopyrite, and supergene minerals in vug replacement? fillings, boxworks, and quartz veins within marble of the Fortymile River assemblage (Werdon et al., 2004a; Dusel-Bacon et al., 2009). A rounded to oval habit of some of the sphalerite and galena grains may reflect the replacement of dolomite fragments (Dusel-Bacon et al., 2009). Drumstick Zn-Pb-Ag skarn?Consists of gossan and relict sulfide grains in weathered surficial materials; grab samples from prospect pits were anomalous in Zn, Pb, Ag, and Au (J. Light, 2008, written commun.; Dusel-Bacon et al., 2009). Drilling determined that mineralization was limited and spotty (Full Metal Minerals, USA, Inc., 2009, proprietary data). Oscar Cu-Zn-Pb-Ag skarn Mineralization occurs in multiple layers of calcic and dolomitic marble in a sequence of amphibolite facies quartzite, hornblende gneiss, felsic metaigneous rocks, and minor quartz-mica schist of the Fortymile River assemblage (Dusel-Bacon et al., 2009). *Letter given where unlabeled in Fig. 3.

of amphibolite in the Fortymile River assemblage that is bounded by north- Northwest Southeast east-trending faults west of the Mount Veta ridge (Dusel-Bacon et al., 2013; Day et al., 2014). The metaharzburgite lens is too small to show at the scale of map, but is located due east of the Drumstick mineral prospect shown in Figure 3. An east-dipping, low-angle fault (~15°–20° dip), inferred to be a thrust fault on the basis of higher grade rocks in the hanging wall relative to the footwall of the fault, places rocks of the Chicken metamorphic complex onto those of the Nasina assemblage (Full Metal Minerals, USA, Inc., 2008, in-house report; 20- >300m Day et al., 2014) (Figs. 3 and 4). 210 ± 3 Ma 1

Base and Precious Metal Mineral Prospects ~ 30-80m

Epigenetic base and precious metal prospects in the Fortymile district occur within a northeast-trending 90-km-long belt that extends from skarn and porphyry prospects associated with the Mount Harper batholith near the western boundary of the Eagle quadrangle (prospects A-C, and Section 21 prospect; Fig. 2; Table 1) (Werdon et al., 2004a) through skarn, epithermal 188 ± 5 Ma base metal veins, and Pb-Zn-Ag carbonate-replacement-style massive sulfide in the Mount Veta area (Figs. 3 and 4; Table 1) (Dusel-Bacon et al., 2009; Siron et al., 2010; Full Metal Minerals, USA, Inc., 2012, in-house report) to skarn and associated Pb-Zn-Ag carbonate replacement mineralization of the Lead Creek 2 and Champion Creek prospects near the eastern boundary of the quadrangle (prospect B; Fig. 2) (Dusel-Bacon et al., 2003). The 40Ar/39Ar dating at prospects in the Mount Harper batholith (Newberry et al., 1998a; Newberry, 2000) yielded a muscovite plateau age of 102.7 ± 0.4 25 50 Meters Ma from a quartz-wolframite-muscovite vein in a quartz porphyry-aplite plug at the Section 21 prospect and a biotite plateau age of 94.2 ± 0.3 Ma from Upper Metasedimentary Dolomitized Marble Marble Breccia a granitic body within 100 m of the Lucky 13 W-rich skarn on Larsen Ridge Sequence (prospect A, Fig. 3), interpreted to date vein mineralization within the plug, and Lithic-quartzite Quartz-diorite Felsic Porphyry Dike skarn mineralization, respectively. (Kechumstuk Fault) Meta-volcaniclastic Marble Massive Sulfide During 2006–2012, intensive mineral exploration for base and precious met- Footwall Rocks als was conducted by Full Metal Minerals, USA, Inc., and Full Metal Zinc, Ltd. in the Mount Veta area (Fig. 4) in the western Fortymile mining district. Exploration Figure 5. Generalized cross section and structural stratigraphic column for Little Whiteman pros- was focused on seven prospects, with most of the drilling and exploration done pect defined by mapping and logging of more than 13,343 m of drill core in 60 drill holes (com- pleted as of 2008). Box in column outlines the main mineralized interval. Modified from Siron on the Little Whiteman (LWM) prospect. The LWM and the Mitchell, Little Enchi- et al. (2010). Sensitive high-resolution ion microprobe U-Pb ages are shown for quartz diorite lada, West LWM, and Fish prospects (Table 1) are spatially associated with the unit (1) sampled at locality 32 (data from Dusel-Bacon et al., 2009), and felsic porphyry dike (2) northeast-trending Kechumstuk fault zone east of the Mount Veta ridge. The Eva sampled at map number 7 (Fig. 3) (data published in this paper). Creek, Drumstick, and Oscar prospects (Table 1) are associated with smaller northeast-trending faults west of the Mount Veta ridge (Figs. 3 and 4) (Day et al., 2014). The LWM prospect (Siron et al., 2010) consists of steeply southeast-dip- entations, observations from LWM drill core, and geologic mapping, indicate ping Pb-Zn-Ag ± Cu massive and semimassive sulfide chimneys and man- both northwest-side-up normal dip-slip and sinistral strike-slip displacement tos that replace marble bodies in the greenschist facies Nasina assemblage. in this region of the Kechumstuk fault (Siron et al., 2010). The largest sulfide Sulfide replacement occurs in the hanging wall of the northeast-trending, replacement bodies are found within ~25 m of the Kechumstuk fault. An air- southeast-dipping Kechumstuk fault, adjacent to subsidiary fault splays, and borne electromagnetic-resistivity-magnetic survey (Burns et al., 2008) and a at the contacts of steeply southeast-dipping, sericitically altered felsic por- ground magnetic susceptibility survey, geologic mapping, and drill core log- phyry dikes (Fig. 5). Structural interpretations, based on synthesis of dike ori- ging (Siron, 2010) suggest that the Kechumstuk fault zone has a left-lateral jog

Dusel-Bacon, C., Aleinikoff, J.N., Day, W.C., and Mortensen, J.K., 2015, Mesozoic magmatism and timing of epigenetic Pb-Zn-Ag mineralization in the western Fortymile mining district, east-central Alaska: Zircon U-Pb geochronology, whole-rock geochemistry, and Pb isotopes: Geosphere, v. 11, doi:10.1130/GES01092.1. Supplemental File 1. Map numbers, sample numbers, location information, and detailed petrographic description of each SHRIMP U-Pb zircon sample from the Fortymile district, east-central Alaska. that forms a rhomboidal pull-apart releasing bend in the vicinity of the LWM Late Triassic Intrusive Rocks (216–208 Ma) Rock name Accessory SHRIMP U- Latitude in Longitude in Alteration Map Location / (minerals listed in Qtz Kfs Hbl Ms minerals Sample No. Pb zircon decimal decimal Texture Pl (%)Bt (%) Cpx (%) (degree; No. body name decreasing (%) (%) (%) (%) (decreasing age1 degree degree minerals) abundance) abundance prospect (Siron et al., 2010). Zircon from a quartz diorite sill that overlies the Late Triassic 1 09AD-319 215.5 ± 3.4small body E of 64.11043 -143.18031 Metamorphically Medium-grained, hypidiomorphic granular, with 15 35 40 Ttn, Fe-Ti oxide, Locally strong Mt. Veta overprinted Hbl Qtz weakly developed foliation defined by 1-3 (up to Ap, Zrn Hbl to Ep and intrusion diorite 7) mm Hbl laths. Greenschist facies alteration Chl; Pl to Ser locallized in shear bands (green-schist facies sulfide bodies at the LWM yielded a Late Triassic (210 ± 3 Ma) SHRIMP U-Pb All Late Triassic rock samples (1–4, Figs. 6A–6E) are medium-grained, hyp- minerals)

208ADb24 215.0 ± 3.5pluton of 64.089167 -143.03778 Hbl-Bt granodioriteMedium-grained, hypidiomorphic granula; 20 5555 15 TraceMag, Ilm, Ttn, Zrn, Moderate; Hbl Kechumstuk weakly developed local alignment of Hbl laths Ap to Bt, Chl and Mtn. Fe-Ti oxide; Fs to clay zircon age (Dusel-Bacon et al., 2009) (31 in Fig. 3). Siron et al. (2010) proposed idiomorphic granular with a weakly developed foliation defined by alignment 379AFr2005 212.0 ± 3.3Taylor Mtn. 63.98111 -142.15222 Hbl-Bt Qtz Medium-grained, hypidiomorphic granula; 10 5601015Ttn, Mag, Ep, ZrnMinor; Hbl to batholith monzodiorite weakly developed local alignment of Hbl laths Ep or Zo

409AD-338 207.9 ± 2.9small body 64.10511 -143.11206 Foliated Hbl Medium-grained, hypidiomorphic granular, 40 54510Ttn, Zrn, Ap Minor; cores that mineralizing hydrothermal fluids were channeled along the Kechumstuk of hornblende laths, as well as biotite in samples 2 and 3 (Supplemental File 1 within Mt. Veta leucotonalite foliation defined by trains of 1 mm Hbl laths and of largest (2 intrusion elongate patches of polygonized and strained mm) Pl to quartz; Kfs in interstitial sub-mm patches; clay; Hbl to euhedral Ttn to 0.5 mm Act Early Jurassic 509AD-240 190.5 ± 4.8 small body W of 64.18772 -143.13098 Foliated, altered Foliated, medium-grained, hypidiomorphic 20 35 40 Ttn, Zrn, Ap Intense; all fault and were vertically restricted by the quartz diorite sill. [see footnote 1]). Two of the samples are from large intrusions: sample 2 (215.0 with ~1.0 – Mt. Veta leucogranite granular, highly altered granitoid. Kfs mostly as mafic grains 2.0 Ga intrusion interstitial masses and euhedral Pl is mostly to Chl, Ep and inherited altered to Ser. Trains of strained and Zo; most Pl to cores polygonized Qtz and fibrous Chl (likely former Ser and minor Bt) define the foliation. Ep 6LWM 07-09- 187.7 ± 2.3sill in LWM drill 64.23379 -142.82556 Altered Bt-bearing Fine-grained, carbonate- and Ser-altered rock 50 240 3ApIntense; Pl to The Eva Creek Ag-Zn-Pb-Cu prospect consists of sulfides and supergene ± 3.5 Ma) is a hornblende-biotite granodiorite from the pluton of Kechumstuk 208 core felsic dike containing thin, 0.5-1 mm long, aligned Bt Ser and Ab; books with Fe-rich margins and carbonate Bt to interiors. Fine-grained Qtz-Fs matrix carbonate interspersed with alteration Ser. Carbonate veins cut sample.

708ADb22 186.2 ± 3.0 Mt. Veta 64.093-143.1458Hbl Qtz monzoniteKfs present as megacrysts (0.5-2 cm, typically 10 40 30 20 TtnMinor; Hbl to minerals in vug fillings, boxworks, and quartz veins within marble (Werdon et Mountain (Figs. 3 and 4) and sample 3 (212.0 ± 3.3 Ma) is a hornblende-biotite intrusion 1 cm but up to 5 cm in length) in a medium- Chl or Ep grained matrix of Hbl, Kfs, Pl, and Qtz. Moderately to strongly foliated. Myrmekite partially rims K-spar megacrysts. 808AD-100 184.8 ± 2.9dike W of Mt. 64.1016 -143.1905Cpx granodiorite Fine-grained, hypidiomorphic granular texture 30 530Trace 35 Ttn, Zrn, Ap Minor; Pl to Zo- al., 2004a; Dusel-Bacon et al., 2009). Metal anomalies in soils at the prospect quartz monzodiorite from the Taylor Mountain batholith in the eastern part of Veta intrusion dike with weakly developed foliation defined by Czo and clay aligned Cpx prisms and elongate patches of strained and polygonized Qtz are elongated in a northeast-southwest direction (Dashevsky et al., 1986), par- the Fortymile district (Fig. 2). The other two Late Triassic samples (1 and 4 in Fig. allel to the faults mapped in the area (Fig. 4). A drill hole penetrated pyrite- and 3) are from small intrusive bodies: sample 1 (215.5 ± 3.4 Ma) is from a small 1 Supplemental File 1. Map numbers, sample num- magnetite-bearing biotite schist and calcite-filled breccia zones as much as 17.7 body of weakly foliated and metamorphosed hornblende quartz diorite (Figs. bers, location information, and detailed petrographic description of each SHRIMP U-Pb zircon sample m thick (Dashevsky et al., 1986). 3, 4, and 6A) at the base and west of the Mount Veta intrusion and sample 4 employed in the study of the Mesozoic igneous The Drumstick Zn-Pb-Ag prospect, located 2.2 km north of the Eva Creek (207.9 ± 2.9 Ma) is a foliated hornblende leucotonalite from a small body within rocks from the Fortymile district, east-central Alaska. prospect, consists of thin, spotty mineralized rock in discontinuous marble the Mount Veta intrusion (Figs. 3, 4, and 6D). Please visit http://dx.doi.org/10.1130/GES01092.S1 or the full-text article on www.gsapubs.org to view beds and breccias within a section predominantly composed of quartz-mica Supplemental File 1. schist and amphibolite (Full Metal Minerals, USA, Inc., 2009, in-house report). The Oscar prospect is composed of several Cu-Zn-Pb-Ag skarns within mar- Early Jurassic Intrusive Rocks (191–181 Ma) ble interlayered with quartzite, gneiss, and schist of the Fortymile River assemblage (Werdon et al., 2004a; Full Metal Minerals, USA, Inc., 2007, in-house report; Early Jurassic rock samples (5–11, Figs. 6F–6L) vary considerably in tex-

Dusel-Bacon, C., Aleinikoff, J.N., Day, W.C., and Mortensen, J.K., 2015, Mesozoic magmatism and timing of epigenetic Pb-Zn-Ag mineralization in the western Fortymile mining district, east-central Dusel-Bacon et al., 2009). The skarns occur in proximity to both the Early Juras- ture and mineralogy (Supplemental File 1 [see footnote 1]). Foliation is mod- Alaska: Zircon U-Pb geochronology, whole-rock geochemistry, and Pb isotopes: Geosphere, v. 11, doi:10.1130/GES01092.1. sic Mount Veta intrusion and a Late Cretaceous intrusion dated in our study. erately to well developed in all but one sample (10, Fig. 6K) and is defined by Supplemental File 2. SHRIMP U-Pb zircon data from the Fortymile district, east-central Alaska. Part A. Description of analytical methods. Mapped structures at the property include prospect-scale folds and northeast- aligned hornblende laths (e.g., Figs. 6H, 6J), K-feldspar megacrysts (e.g., Fig. U-Pb geochronology of zircon was performed using the Stanford–U.S. Geological Survey

sensitive high resolution ion microprobe-reverse geometry (SHRIMP-RG). About 1-2 kg of rock and north-south–trending faults (Full Metal Minerals, USA, Inc., 2007, in-house 6L), and less commonly by biotite books (Fig. 6G), trains of strained and poly- was collected for each dated sample. Zircon was extracted using standard mineral separations report). The prospect occurs within 1 km of a northeast-trending normal fault. gonized quartz (e.g., Fig. 6F), and clinopyroxene prisms (Fig. 6I). The Mount techniques, including crushing, pulverizing, Wilfley table, magnetic separator, and heavy liquids. Individual grains were hand picked, mounted in epoxy, ground to approximately half-thickness Veta intrusion yielded ages of 186.2 ± 3.0, 184.5 ± 3.0, and 181.2 ± 2.6 Ma to expose internal zones, and sequentially polished using 6 µm and 1 µm diamond suspension.

All grains were imaged digitally in transmitted and reflected light, and in cathodoluminescence (Table 2) and forms a northeast-striking elongate, composite body (Fig. 3). (CL) using the scanning electron microscope (SEM). RESULTS Two zircon samples were collected from small, locally deformed granitoids SHRIMP analysis (following the methods of Williams, 1998) consisted of excavating a pit

about 25-35 µm in diameter and about 1 µm in depth, using a primary ion beam at a current of that intrude metasedimentary rocks of the Fortymile River assemblage west about 5-7 nA. The magnet cycled through the mass stations 5 times per analysis. Raw data were Zircon U-Pb Samples and SHRIMP Ages of, and at the structural base of, the synkinematic Mount Veta intrusion (Day reduced using Squid 2 (Ludwig, 2009) and plotted using Isoplot 3 (Ludwig, 2003). Measured 206Pb/238U ratios were normalized to values obtained for standard zircon R33 (419 Ma; Black et et al., 2014) (Fig. 4): a foliated leucogranite body (5, Fig. 6E) and a clinopy- al., 2004). Uranium concentrations are believed to be accurate to ±20%. U-Pb data are plotted

on Tera-Wasserburg concordia plots (Supplemental File 2, Part B) to visually identify coherent Zircon U-Pb geochronology was performed using the Standford–U.S. Geo- roxene-bearing granodiorite dike (8, Fig. 5I). Zircon from these two samples ages groups. Error ellipses and error bars are shown with 2-sigma uncertainties. Weighted logical Survey SHRIMP-RG (reverse geometry) on 23 samples of intermediate yielded ages of 190.5 ± 4.8 and 184.8 ± 2.9 Ma, within the uncertainty of zircon averages of selected 206Pb/238U ages were calculated to obtain an age for each sample; exclusions

of data from the age calculations were based on the statistical methodology of Isoplot 3. Isotopic to felsic igneous rocks from the western Fortymile district in the Eagle quad- ages from the Mount Veta intrusion. rangle (Fig. 3) and on three samples from the eastern part of the district in Two tabular intrusive bodies just east of the Mount Veta intrusion were

1 the adjacent Tanacross quadrangle (Fig. 2). The samples were collected from also dated. Sample 6 is a fine-grained, carbonate- and sericite-altered felsic 20 separate plutons, stocks, or dikes. Table 2 presents a summary of the U-Pb dike (Fig. 6G) from drill core of the LWM carbonate replacement prospect geochronology. More details about each sample can be found in Supplemen- (Figs. 3 and 5). Based on compositional similarities, we correlate sample 6 2 Supplemental File 2. SHRIMP U-Pb zircon data from tal Files 11 and 22 (Part A, Description of analytical methods; Part B, Figures with steeply southeast-dipping sheeted felsic porphyry dikes that are concor- the Fortymile district, east-central Alaska. Part A de- scribes the analytical methods used for U-Pb geo- showing representative cathodoluminescence (CL) images of zircon, concordant with elongate marble bodies. The sheeted dikes crosscut the ca. 210 Ma chronology of zircon. Part B presents representative dia plots, and weighted average plots; and Part C, Interpretations of the an- quartz diorite intrusion and are spatially associated with sulfide replacement cathodoluminescence images of zircon, concordia alytical results), and Supplemental File 33 (SHRIMP U-Th-Pb data). Samples in the hanging wall of the northeast-trending, southeast-dipping, sinistral plots, and weighted average plots. Part C presents information used in the interpretation for analytical are referred to by 1–28 (map numbers), according to decreasing age, that cor- strike-slip and normal dip-slip Kechumstuk fault (Fig. 5). Although all dated results for each dated sample, including the morphol- respond with map numbers in the text, map figures, and tables. U-Pb zircon zircon grains in sample 6 have similar concentric oscillatory zoning in cath- ogy and cathodoluminescence zoning characteristics age uncertainties are 2σ. Previously published SHRIMP U-Pb zircon ages also odoluminescence (CL), two distinct age groups were found: one group (n = 5) of its zircon. Please visit http://dx.doi.org/10.1130 206 238 /GES01092.S2 or the full-text article on www.gsapubs are shown in Fig. 3 for map numbers 19 and 20 (Bacon et al., 2014) and 29–31 yielded a weighted average Pb/ U age of 187.7 ± 2.3 Ma, whereas the other .org to view Supplemental File 2. (Dusel-Bacon et al., 2009). group (n = 7) yielded a weighted average 206Pb/238U age of 177.9 ± 1.1 Ma. We

Supplemental File 3. SHRIMP U-Th-Pb data for zircon from rocks of the Fortymile district, east-central Alaska.

measured measured % common 206 238 1 1! error 238 206 2 1! error 207 206 2 1! error analysis no. 204 206 207 206 U (ppm) Th/U Pb/ U (Ma) U/ Pb Pb/ Pb Pb/ Pb Pb/ Pb 206Pb (Ma) (Ma) (Ma) Map. No. 1, sample no. 09AD-319 (hornblende quartz diorite at base of the Mount Veta pluton) Map number Sample SHRIMP U-Pb zircon age (Ma)Geologic body from which sample was collectedRock name* 09AD319-1.10.0000430.0491 -0.145 403 0.806 205 3.331.0 1.6 0.0484 2.6 09AD319-2.10.0000640.0487 -0.203 519 0.900 212 3.330.1 1.6 0.0478 2.4 09AD319-3.10.0000860.0498 0.001 261 0.042 187 7.734.0 4.1 0.0486 6.1 09AD319-3.20.0000000.0501 -0.037 212 0.605 215 4.629.4 2.2 0.0501 2.9 09AD319-4.10.0001320.0474 -0.366 243 0.472 2114.3 30.2 2.1 0.0455 4.2 09AD319-4.2-0.000355 0.0484 -0.225 125 0.220 206 10.330.6 5.1 0.0536 8.0 09AD319-5.10.0000000.0515 0.140 299 0.495 213 4.429.7 2.1 0.0515 2.8 Late Triassic 09AD319-6.10.0000770.0524 0.225 213 0.594 224 10.428.3 4.7 0.0513 3.7 09AD319-6.20.0002600.0457 -0.550 150 0.112 202 4.931.8 2.5 0.0419 7.9 09AD319-7.10.0002080.0473 -0.412 261 0.522 222 4.528.8 2.1 0.0442 5.1 09AD319-7.20.0000220.0516 0.128 825 0.002 221 3.828.7 1.7 0.0513 1.7 09AD319-8.10.0000340.0500 -0.056 457 0.718 214 3.929.7 1.8 0.0495 2.2 09AD319-8.2-0.000079 0.0496 -0.150 225 0.055 230 4.927.6 2.1 0.0507 3.7 1 09AD-319 215.5 ± 3.4 Small body west of Mount Veta intrusion Metamorphically overprinted Hbl Qtz diorite 09AD319-9.10.0001910.0520 0.192 376 0.565 217 4.129.2 1.9 0.0492 3.8 09AD319-9.20.0009240.0508 0.090 389 0.010 200 3.832.3 2.0 0.0370 9.9 09AD319-10.1 -0.000088 0.0509 0.037 206 0.522 221 7.028.7 3.2 0.0521 3.9 09AD319-10.2 0.0000000.0499 -0.053 138 0.198 2115.5 30.1 2.6 0.0499 3.9 09AD319-11.1 0.0000000.0500 -0.055 373 0.536 217 4.129.3 1.9 0.0500 2.4 2 08ADb24 215.0 ± 3.5 Pluton of Kechumstuk Mountain Hbl Bt granodiorite Map. No. 2, sample no. 08ADb24 (hornblende-biotite granodiorite from pluton of Kechumstuk Mountain) 08ADb24-1.10.0000000.0500 -0.033 131 0.29 206 9.431 4.6 0.0500 3.6 08ADb24-2.10.0000000.0482 -0.238 156 0.38 200 2.932 1.4 0.0482 3.3 08ADb24-3.1-0.0001180.0470 -0.473 125 0.25 231 4.527 2.0 0.0487 5.1 08ADb24-4.10.0001290.0517 0.165 1170.25 212 4.430 2.1 0.0498 8.9 08ADb24-5.1-0.000208 0.0495 -0.120 77 0.24 216 5.129 2.4 0.0526 7.4 3 79AFr2005 212 ± 3.3 Taylor Mountain batholith Hbl Bt Qtz monzodiorite 08ADb24-6.10.0002810.0469 -0.447 179 0.43 220 4.029 1.9 0.0428 6.8 08ADb24-7.10.0008980.0582 1.027 238 0.71 192 3.333 1.8 0.0449 9.4 08ADb24-7.2-0.000170 0.0509 0.084 106 0.36 206 4.431 2.1 0.0534 6.3 08ADb24-8.10.0001880.0491 -0.160 162 0.45 213 4.030 1.9 0.0463 5.5 08ADb24-9.1-0.000055 0.0491 -0.174 255 0.55 217 4.929 2.3 0.0499 2.9 08ADb24-10.1 0.0001290.05110.097 125 0.37 2116.7 30 3.2 0.0492 5.6 4 09AD-338 207.9 ± 2.9 Small body within Mount Veta intrusion Foliated Hbl leucotonalite 08ADb24-11.1 0.0000000.0521 0.250 101 0.28 199 3.332 1.6 0.0521 4.1 08ADb24-12.1 0.0002140.0469 -0.456 132 0.27 219 4.229 2.0 0.0437 6.4 08ADb24-13.1 -0.000336 0.0467 -0.470 95 0.25 216 4.629 2.2 0.0516 7.9 08ADb24-13.2 0.0000000.0521 0.241 90 0.37 202 3.331 1.6 0.0521 4.0 08ADb24-14.1 -0.000029 0.0491 -0.182 557 0.09 219 4.029 1.8 0.0495 2.0

Map. No. 3, sample no. 79AFr2005 (hornblende-biotite quartz monzodiorite from Taylor Mountain batholith) Early Jurassic 79AFR2005-1.10.0001630.0524 0.27 98 0.4 205 2.730.9 1.3 0.0500 7 79AFR2005-2.10.0000780.0508 0.05 399 0.5 215 3.029.5 1.4 0.0496 3 79AFR2005-3.10.000511 0.0481 -0.27 125 0.3 207 2.631.0 1.3 0.0405 11 79AFR2005-4.10.0002380.0487 -0.22 195 0.4 213 3.629.9 1.7 0.0451 6 79AFR2005-5.10.0008530.0520 0.19 105 0.2 216 8.129.7 3.8 0.0393 14 79AFR2005-6.10.0002400.0513 0.10 285 0.5 217 3.329.3 1.5 0.0478 4 5 09AD-240 190.5 ± 4.8 with ca.1.0–2.0 Ga Small body west of Mount Veta intrusion Foliated, altered leucogranite 79AFR2005-7.1-0.000063 0.0500 -0.06 251 0.3 216 5.929.3 2.8 0.0509 3 79AFR2005-8.10.0000000.0479 -0.34 251 0.4 226 3.528.1 1.6 0.0479 3 79AFR2005-9.10.0000000.0513 0.08 145 0.3 226 4.328.0 1.9 0.0513 4 79AFR2005-10.1 0.0004060.0515 0.12 111 0.3 218 4.429.3 2.1 0.0455 9 79AFR2005-11.1 0.0004310.05110.09 183 0.3 209 6.430.6 3.1 0.0447 7 inherited cores 79AFR2005-12.1 0.0007840.0531 0.38 127 0.3 196 3.932.7 2.0 0.0414 12 79AFR2005-13.1 0.000112 0.0465 -0.49 152 0.4 215 4.029.7 1.9 0.0449 5 79AFR2005-14.1 -0.000289 0.0516 0.19 1100.2 196 2.532.1 1.3 0.0558 6

Map. No. 4, sample no. 09AD-338 (foliated hornblende leucotonalite from small body within Mount Veta intrusion) 09AD338-1.10.0000000.0499 -0.047 283 0.48 209 2.230 1.1 0.0499 2.3 6LWM 07-09-208 187.7 ± 2.3 Felsic dike in Little Whiteman (LWM) drill core Altered Bt-bearing felsic dike 09AD338-2.10.000119 0.0496 -0.086 123 0.28 209 13.330 6.4 0.0479 5.3 09AD338-3.1-0.000100 0.0517 0.185 170 0.29 206 2.531 1.2 0.0532 4.3 09AD338-4.1-0.000035 0.0495 -0.098 515 0.68 208 2.730 1.3 0.0500 2.2 09AD338-5.10.0000510.0499 -0.083 322 0.30 220 6.529 3.0 0.0491 2.9 09AD338-6.1-0.000177 0.0531 0.342 202 0.30 213 4.030 1.9 0.0557 4.4 09AD338-7.10.0000000.0498 -0.075 434 0.27 216 2.929 1.4 0.0498 2.0 7 08ADb22 186.2 ± 3.0 Mount Veta intrusionHbl Qtz monzonite 09AD338-8.10.0001210.0515 0.157 364 0.31 204 2.231 1.1 0.0497 4.4 09AD338-9.1-0.000065 0.0489 -0.196 237 0.30 219 3.629 1.6 0.0499 3.3 09AD338-10.1 0.0001010.0492 -0.129 312 0.19 206 3.131 1.5 0.0477 3.3 09AD338-11.1 0.0000000.0517 0.187 362 0.39 206 2.231 1.1 0.0517 4.3 09AD338-12.1 -0.000051 0.0507 0.007 250 0.54 222 3.528 1.6 0.0514 5.3 8 08AD-100 184.8 ± 2.9 Dike west of Mount Veta intrusionCpx granodiorite dike Map. No. 5, sample no. 09AD-240 (foliated leucogranite) 09AD240-1.10.0000190.0506 0.083 18620.03 191 2.033 1.1 0.0503 1.1 09AD240-1.20.0000000.1052 1.014 286 0.42 1578 17.041.1 0.1052 0.6 09AD240-2.10.0000000.0496 -0.057 12860.01 196 2.332 1.2 0.0496 1.5 09AD240-3.10.0000000.0498 0.007 14 0.01 183 21.43511.8 0.0498 14.0 9 8A5-4 184.5 ± 3.0 Mount Veta intrusionFoliated Hbl Cpx monzonite 09AD240-3.20.0000060.1348 2.000 495 0.11 1958 26.831.3 0.1347 0.4 09AD240-4.10.0005280.0442 -0.774 36 0.00 213 7.830 3.8 0.0363 24.2 09AD240-5.10.0000000.0522 0.324 40 0.01 178 25.43614.4 0.0522 8.1 09AD240-5.2-0.000023 0.0743 2.779 553 0.06 285 3.721 1.3 0.0746 1.4 09AD240-6.10.0000000.0505 0.078 761 0.01 187 2.334 1.2 0.0505 3.2 09AD240-6.1-0.000010 0.0507 0.107 18110.02 185 2.734 1.5 0.0508 1.1 10 10AD368 183.4 ± 3.6 Sill in hanging wall of west-vergent thrust, east of Altered Kfs megacrystic Hbl Bt granite 09AD240-7.10.0000000.0520 0.075 24540.07 259 2.724 1.1 0.0520 0.8 09AD240-9.10.0010660.0561 0.803 21 0.00 180 17.53610.0 0.0403 43.0 09AD240-9.20.0000510.1611 10.871 387 0.62 111111.351.0 0.1604 0.5 09AD240-10.1 -0.000964 0.0576 0.936 23 0.00 197 9.231 4.9 0.0714 20.6 09AD240-11.1 -0.000012 0.0502 0.039 13930.01 191 2.133 1.1 0.0504 1.2 Kechumstuk fault zone porphyry 09AD240-12.1 -0.001007 0.0529 0.438 21 0.00 165 34.73821.2 0.0674 22.7 09AD240-13.1 0.0010820.0476 -0.269 10 0.00 183 10.836 6.3 0.0313 56.1 09AD240-13.2 0.0000000.0749 0.311 355 0.29 998 13.961.4 0.0749 0.8 11 08ADb08 181.2 ± 2.6Mount Veta intrusion Kfs megacrystic Hbl Qtz monzonite

1 Mid-Cretaceous 12 08ADb20 111.8 ± 1.5 Corner granite Leucogranite 13 08AD-052 108.8 ± 1.7Corner granite Altered Bt granite 3Supplemental File 3. SHRIMP U-Th-Pb data for zircon from rocks of the Fortymile district, east-central Alaska. 14 10ADb23 103.2 ± 1.5Mount Harper batholith Bt granite Please visit http://dx.doi.org/10.1130/GES01092 .S3 or 15 10ADb26A 101.4 ± 1.4 Dike cutting Mount Harper batholith Aplite dike cut by Qtz veinlets the full-text article on www.gsapubs.org to view Sup- 16 10ADb25 96.2 ± 1.3 Dike cutting Mount Harper batholith Qtz Pl granodioritic porphyry dike plemental File 3. 17 09AD-343 95.8 ± 1.5 Porphyry intrusion cutting Corner granite Altered Qtz Fs Bt granitic porphyry 18 08AD-032A 93.9 ± 1.3 Small porphyry within splays of Kechumstuk fault zone Altered Qtz Pl rhyolite porphyry Late Cretaceous 21 118A5 68.1 ± 0.8 Dike within Kechumstuk fault near Mitchell prospect Qtz Kfs rhyolite porphyry 22 81A5 67.9 ± 1.1 Intrusion north of Mount Veta Bt Hbl granite 23 129A5 67.7 ± 0.7 Intrusion between splays of Kechumstuk fault zone Fine-grained Bt granite northof Mitchell prospect 24 08ADb14 67.0 ± 1.5 Mount Fairplay Ksp megacrystic Cpx Hbl Bt syenite 25 08ADb13 66.5 ± 1.1 Mount FairplayEquigranular Hbl Bt Qtz monzonite 26 08ADb19 65.8 ± 1.5 Intrusion north of Mount Veta Bt Hbl granite 27 08ADb03 65.8 ± 1.4 Intrusion north of Mount Veta Bt granite 28 140A5 ca. 65 or younger Dike within splay of Kechumstuk fault zone and cutting Fine-grained microporphyritic aplite pluton of Kechumstuk Mountain Note: Map numbers refer to text Figure 3. Sample details and data are provided in Supplemental Files 1, 2, and 3 (see text footnotes 1, 2, and 3). SHRIMP—sensitive high-resolution ion microprobe. *Minerals listed in decreasing abundance. Abbreviations: Bt—biotite; Cpx—clinopyroxene; Fs—feldspar; Hbl—hornblende; Kfs—K-feldspar; Pl—plagioclase; Qtz—quartz.

79AFr2005 A 09AD319 B 08ADb24 C 3 1 2

1 cm 0.5 cm 2 mm 216±3 Ma 215±4 Ma 212±3 Ma

D 09AD338 E 09AD240 F 09AD240 4 5 5

1 cm 1 cm 2 mm 208±3 Ma 191±5 Ma 191±5 Ma

G LWM07-09-208 H 08ADb22 I 08AD100 6 7 8

2 mm 1 cm 0.5 cm 188±2 Ma 186±3 Ma 185±3 Ma

J 8A5-4 K 10AD368 L 08ADb08 9 10 11

0.5 cm 2 mm 4 cm 184±3 Ma 183±4 Ma 181±3 Ma

Figure 6. Photographs and photomicrographs of Triassic and Jurassic samples discussed in text. (A–L) Map numbers (circled) of dated samples and zircon U-Pb ages (rounded to tens) are shown in Figures 2 or 3. Unrounded U-Pb ages, petrographic descriptions of dated samples, and sample location information are given in Supplemental File 1 (see footnote 1).

interpret the older age as the time of crystallization of the dike (see Supple- to cut both the quartz monzonite and aplite dike at the Section 21 prospect and mental File 2, Part A, Fig. F of Part B, and Part C [see footnote 2]) and suggest yielded a crystallization age of 96.2 ± 1.3 Ma. that the dike was reheated ca. 178 Ma. This interpretation is based on the Two small porphyry intrusions in the southern part of the Kechumstuk fault facts that the dated sample occurs within ~5 km of a megacrystic phase of zone also yielded zircon U-Pb ages in the younger phase of mid-Cretaceous the Mount Veta intrusion that is 181 ± 3 Ma (sample 11) and the zircon U-Pb magmatism: (1) sample 17 (95.8 ± 1.5 Ma) is a quartz-feldspar-biotite rhyolite crystallization age of the felsic dike is in agreement with Ar-Ar dating (Sup- porphyry (Fig. 7F) that intrudes the Corner granite, and (2) sample 18 (93.9 ± plemental File 44). Incremental heating of secondary sericitic muscovite from 1.3 Ma) is quartz-plagioclase rhyolite porphyry (Fig. 7G) from a small body that another highly altered felsic porphyry dike from a different drill hole yielded a crops out between splays of the Kechumstuk fault zone (Figs. 3 and 4). 40Ar/39Ar age of 187.5 ± 2.0 Ma for the most retentive 6 fractions and minor gas loss ca. 65 Ma. The more retentive phase of the sample also had a lower Ca/K ratio than the rest of the gas release, indicative of a more pure muscovite Late Cretaceous Intrusive Rocks (68–66 Ma) mineral phase (Supplemental File 4 [see footnote 4]). Sample 10 is a nonfoliated K-feldspar megacrystic hornblende-biotite gran- Eight Late Cretaceous intrusive rocks dated in this study (samples 21–27, ite porphyry (Fig. 6K) that forms a thin sill in the hanging wall of the thrust Fig. 8) display no evidence of a preferred fabric and document a relatively brief fault that places Chicken metamorphic complex rocks over Nasina assemblage Late Cretaceous magmatic episode that formed equigranular granitic intru- rocks, east of the Mount Veta intrusion and the Kechumstuk fault zone (Figs. 3 sions and porphyritic rhyolite dikes (Figs. 3 and 4) and, in the Mount Fairplay and 4). This sample yielded an igneous crystallization age of 183.4 ± 3.6 Ma. intrusion, K-feldspar megacrystic syenite and equigranular quartz monzonite (Fig. 2). Quartz–K-feldspar rhyolite porphyry (21, Figs. 3 and 8A) collected from a dike parallel to and within a strand of the northeast-trending Kechumstuk Mid-Cretaceous Intrusive Rocks (112–94 Ma) fault zone near the Mitchell prospect (Fig. 4) yielded a crystallization age of 68.1 ± 0.8 Ma; a similar age of 67.7 ± 0.7 Ma was determined for fine-grained biotite Mid-Cretaceous rock samples (12–18, Figs. 7A–7G), in contrast to the Trias- granite (Fig. 8C) from a small intrusion bounded by splays of the Kechumstuk sic and Jurassic samples, show no evidence of a preferred fabric, are discor- fault zone ~2 km north of the Mitchell prospect (23, Figs. 3 and 4). Three phases dant to country rocks, and range from hypidiomorphic granular to porphyritic. of a composite pluton just southeast of the Middle Fork caldera in an inferred

Dusel-Bacon, C., Aleiniko , J.N., Day, W.C., and Mortensen, J.K., 2015, Mesozoic magmatism and timing We conclude that they were intruded after the regional Early Jurassic dyna- uplifted fault block (Fig. 3) were dated: (1) fine-grained biotite-hornblende of epigenetic Pb-Zn-Ag mineralization in the western Fortymile mining district, east-central Alaska: Zircon U-Pb geochronology, whole-rock geochemistry, and Pb isotopes: Geosphere, v. 11, doi:10.1130/GES01092.1. mothermal metamorphism (Dusel-Bacon et al., 2002). The two oldest samples granite with dark gray smoky quartz phenocrysts (22, Figs. 3 and 8B; Granite

Supplemental File 4. 40Ar/39Ar age and compositional data from incremental heating of secondary are leucogranite and intensely altered biotite granite (12 and 13, Figs. 7A and of Gold Bottom Creek unit of Day et al., 2014) is 67.9 ± 1.1 Ma, (2) biotite-horn- sericitic muscovite from altered felsic porphyry dike in LWM drill core. Dated muscovite is from sample LWM-088-56 (.250–.150 mm fraction = 60–100 mesh). Data from P. Layer and J. Benowitz (written commun., 2011; Geochronology Laboratory, University of Alaska Fairbanks). 7B, respectively) from a north-trending, elongate body that intrudes Devonian blende granite is 65.8 ± 1.5 Ma (26, Figs. 3 and 8G), and (3) biotite granite is augen gneiss of the Lake George assemblage in the parautochthonous YTa at 65.8 ± 1.4 Ma (27, Figs. 3 and 8H; Granite of Veta Creek unit of Day et al., 2014). 200 the southern edge of the study area (Figs. 3 and 4). The body crops out along Zircon from a microporphyritic aplite dike (sample 28, Figs. 3 and 8I) that oc- 150 a ridge that includes the vertical angle benchmark (VABM) “Corner,” and was curs within a splay of the Kechumstuk fault zone that cuts the pluton of Kechum- 100 Age in Ma designated the Corner granite by Day et al. (2014). Zircon in samples 12 and 13 stuk Mountain (Fig. 4) appears to be mostly (or possibly entirely) xenocrystic. 50 shows fine oscillatory zoning and yielded crystallization ages of 111.8 ± 1.5 and Zircon grains vary in color, external morphology, and CL zoning patterns. The 0 0.0 0.2 0.4 0.6 0.8 1.0 Fraction of 39Ar Released 206 238 2.0 108.8 ± 1.7 Ma, respectively. Pb/ U ages range from ca. 93 to 64 Ma; the ca. 65 Ma age of the two young-

1.5 The next three oldest samples are from the Mount Harper batholith in the est grains is interpreted to represent the maximum age of emplacement.

1.0 Ca/K western part of the study area (Fig. 3). Sample 14 is a medium-grained, hypid- Late Cretaceous ages also were determined for two samples from the sy- 0.5 iomorphic granular biotite granite (Fig. 7C) that yielded a crystallization age of enite of Mount Fairplay unit in the central Tanacross quadrangle (Foster, 1970)

0.0 0.0 0.2 0.4 0.6 0.8 1.0 103.2 ± 1.5 Ma based on 11 out of 15 analyses; 4 analyses yielded ages of ca. (Fig. 2). Clinopyroxene-hornblende-biotite syenite (24) is characterized by dis- Fraction of 39Ar Released 113–109 Ma. The other two samples are dikes that cut the Mount Harper batho- tinctive gray, aligned, 2–6-cm-long, tabular megacrysts of K-feldspar that make lith at the Section 21 Mo-W porphyry prospect. (1) Sample 15 is a fine-grained up ~30% of the rock (Figs. 8D, 8E). Zircon is medium to dark brown, anhedral aplite dike (Fig. 7D) that contains quartz-molybdenite veins and intrudes a to subhedral, and yielded a crystallization age of 67.0 ± 1.5 Ma. CL imagery and quartz monzonite phase of the Mount Harper batholith. Some of the zircon scanning electron microscopy–energy dispersive spectroscopy analysis reveal

4 40 39 Supplemental File 4. Ar/ Ar age and compositional grains from this sample have extremely high U concentrations (~1200–8970 that most grains are composed of relict, oscillatory zoned baddeleyite (ZrO2) data from incremental heating of secondary seric- ppm) making U-Pb analysis problematic; however, a crystallization age of 101.4 that was partially replaced by zircon (Supplemental File 2, Part B, Fig. V [see itic muscovite from altered felsic porphyry dike in ± 1.4 Ma age was determined for seven analyses of a relatively low U concen- footnote 2]). A sample of medium-grained, equigranular hornblende-biotite LWM drill core. Please visit http://dx.doi.org/10.1130 /GES01092.S4 or the full-text article on www.gsapubs tration (~480–1030 ppm) group of zircons. (2) Sample 16, a medium-grained quartz monzonite (25, Fig. 8F) from a different phase of the syenite of Mount .org to view Supplemental File 4. granodioritic quartz porphyry (Fig. 7E), crops out as a linear dike that appears Fairplay unit gave a crystallization age of 66.5 ± 1.1 Ma for colorless, euhedral

A 08ADb20 B 08AD052 C 10ADb23 12 13 14

0.5 cm 0.5 cm 2 cm 112±2 Ma 109±2 Ma 103±2 Ma D 10ADb26A E 10ADb25 F 09AD343 15 16 17

2 mm 2 mm 1 cm 101±1 Ma 96±1 Ma 96±2 Ma

G 08AD032A H 09AD311 I 09AD311 18 21

0.5 cm 1 cm 94±1 Ma 2 mm 0.1 mm

Figure 7. Photographs and photomicrographs of mid-Cretaceous samples discussed in text. (A–G) Map numbers (circled) of dated samples and zircon U-Pb ages (rounded to tens) are also shown in Figure 3 and in Table 2. Unrounded U-Pb ages, petrographic descriptions of dated samples, and sample location information are given in Supplemental File 1 (see footnote 1). (H–I) Photomicrographs of undated, altered biotite granite from the Corner granite body, 1.5 km north of map number 13. Rock contains euhedral plagioclase and smoky quartz grains, coarse-grained polycrystalline aggregates (H) and ubiquitous granophyric quartz-alkali feldspar intergrowths (I).

A 118A5 B 81A5 C 129A5 21 22 23

1 cm 2 mm 68±1 Ma 1 cm 68±1 Ma 68±1 Ma

D 08ADb14 E 08ADb14 F 08ADb13 24 24 25

3 cm 0.5 cm 0.5 cm 67±2 Ma 67±2 Ma 67±1 Ma

G 08ADb19 H 08ADb03 I 140A5 26 27 28

0.5 cm 0.5 cm 0.25 mm 66±2 Ma 66±1 Ma ≤~65 Ma

Figure 8. Photographs and photomicrographs of Late Cretaceous samples discussed in text. (A–I) Map numbers (circled) and zircon U-Pb ages of dated samples are also shown in Figures 2 or 3 and Table 2. Unrounded U-Pb ages, petrographic descriptions of dated samples, and sample location information are given in Supple- mental File 1 (see footnote 1). (E–H) In stained rock slabs, K-feldspar is stained yellow and plagioclase is stained pink.

zircon (Supplemental File 2, Part B, Fig. W [see footnote 2]), which overlaps that cause them to plot either just inside the arc field close to its border with within analytical uncertainty the age for sample 24. the syncollisional granite field or in the corner of the syncollisional granite field (Fig. 9E; Table 3). All mid-Cretaceous samples plot in the corner of the arc field, with the exception of the 112 Ma sample from the Corner granite that plots Whole-Rock Geochemistry in the field of within-plate and anomalous ocean ridge-type granite (Fig. 9E). Primitive mantle-normalized multielement plots for dated samples in each Table 3 provides whole-rock geochemical analyses for Fortymile district Me- of the six Mesozoic age groups are shown in Figures 9F–9K. All of the Forty- sozoic felsic and intermediate-composition granitoids dated by U-Pb geochro- mile igneous samples have low normalized abundances of Nb and Ta relative nology in this study and associated, previously dated or undated, igneous rocks to Th and La, a hallmark of modern arc rocks (Sun and McDonough, 1989). of clear age designation, together with three Mesozoic mafic igneous rocks from Negative Eu and Ti anomalies record the removal of previously crystallized pla- the western Fortymile district. Location information for whole-rock samples is gioclase and Fe-Ti oxides, respectively, from the melt. Trace element patterns given in Supplemental File 55. All samples were analyzed by wavelength-dis- of the four Mesozoic groups show progressively more evolved compositions persive X-ray fluorescence and inductively coupled plasma–mass spectrometry through time, indicated by increasing concentrations of Th and progressively at the GeoAnalytical Laboratory, Washington State University (see Dusel-Bacon more negative slopes of the adjacent incompatible elements to the left of the et al., 2013, for a detailed description of analytical procedures). Ti and Eu anomalies. The flatter multielement pattern and overall lower trace element contents of the Late Triassic samples (Fig. 9F) are consistent with their less evolved major element compositions. Samples from the Taylor Mountain Felsic and Intermediate-Composition Rocks batholith have multielement patterns that overlap those of the Late Triassic rocks in the Mount Veta area. Multielement plots of the dated Early Jurassic Whole-rock geochemical plots of the Mesozoic felsic and intermediate-com- rocks in the Mount Veta area are comparable to those in the eastern Fortymile position samples analyzed in our study, together with analyses of dated Me- area and all have parallel patterns with a moderate range in overall primitive

Supplemental File 5. Location data for whole-rock geochemistry samples of felsic and mafic igneous rocks from al., 2009), are shown in Figure 9, grouped by age. Our petrographic obser- The mid-Cretaceous (Figs. 9H, 9I) and Late Cretaceous (Figs. 9J, 9K) sam- the Fortymile district, east-central Alaska. Geochemical data are given in Table 3 and plotted on Figures 9 or 10 of text.

Map No. or Sample No. Age (Ma) Age 2 Quad 3 Latitude Longitude previous pub.1 vations of sample mineralogy are consistent with the rock names based on ples are the most evolved intrusive rocks. Four compositionally evolved Felsic Rocks 09AD-319 1215.5±3.4 LTrEA A-564.11043 -143.18031 08ADb242215.0±3.5LTr EA A-564.08917 -143.03778 09AD-338 4207.9±2.9 LTrEA A-564.10511 -143.11206 volume percent quartz (Q), orthoclase (Or), and albite (Ab) + anorthite (An) (Fig. mid-Cretaceous granitoid samples differ from the others in these age groups a, bT: 212 Ma loc. is 93ADb01 LTrTA D-363.98889 -142.12917 1.4 km to S 93ADb10aT: 215.7±3.1 LTrEA A-464.03833 -142.82722 LWM-07-06-12431, a210±3 LTrEA A-464.23346 -142.82664 08ADb15LTr TA D-463.99056 -142.12861 10AD-394 LTrEA A-464.09205 -142.98376 9A) calculated from whole-rock analyses using the weight-percent normaliza- in more pronounced negative Eu and Ti anomalies (Fig. 9H): these are the 101 09AD-317 LTrEA A-564.10829 -143.16953 a, eT: 201.0±1.4; Hbl: 93ADb09 EJur EA A-464.14222 -142.68278 197.3±0.7 09AD-240 5190.5±4.8 EJur EA A-564.18772 -143.13098 08ADb227186.2±3.0EJurEA A-564.09306 -143.14583 08AD1008184.8±2.9EJurEA A-564.10167 -143.19056 tion scheme of Streckeisen and Le Maitre (1979). Late Triassic igneous rocks Ma aplite from the dike at the Section 21 prospect in the Mount Harper ba- 8A5-4 9184.5±3.0 EJur EA A-564.05990 -143.17010 10AD-368 10 183.4±3.6EJurEA A-464.20405 -142.82817 08ADb0811181.2±2.6 EJur EA A-464.21444 -142.88417 aZr: 188.4±1.2; T: 91ADb10 EJur EA A-264.16917 -141.55889 187.3±0.6 91ADb35aT: 184.6±2.1 EJur EA A-564.15111 -143.12667 consist of quartz monzodiorite or granodiorite and/or tonalite. Early Jurassic tholith and one dated (112 Ma leucogranite) and two undated samples (quartz 93ADb18aZ: 191.3±0.4EJurEA A-364.24000 -142.19722 98ADb15aT: 186.7±1.6 EJur EA A-164.20000 -141.41083 98ADb30aZ: 193.2±0.4EJurEA B-264.33528 -141.84333 07ADb01A 29, aZ: 187±3EJurEA B-464.25680 -142.70002 00ADb18aZ: 197.6±1.1EJurEA B-164.26756 -141.09450 00ADb41aZ: 197.2±2.5EJurEA B-164.35350 -141.40000 rocks display a wide range of silica-undersaturated to silica-saturated compo- feldspar porphyry dike sample 08AD054 and sample 09AD311) from the Corner 08AD036EJurEA A-564.05333 -143.15167 08AD039EJurEA A-464.21472 -142.88472 08ADb18A EJur EA A-564.23000 -143.09167 09AD-229 EJur EA A-564.13062 -143.18312 10AD-402 EJur EA A-564.01826 -143.11465 sitions (monzonite, quartz monzonite, quartz monzodiorite, granodiorite and/ granite intrusion. The leucogranite has the highest primitive mantle-normal- 08ADb02EJurEA B-164.35222 -141.39750 08ADb2012 111.8±1.5 mCret EA A-564.00889 -143.30972 08AD05213108.8±1.7 mCret EA A-564.02861 -143.24639 10ADb2314103.2±1.5 mCret EA A-664.23833 -143.81028 10ADb26A 15 101.4±1.4 mCret EA A-664.20889 -143.79833 10ADb251696.1±1.1 mCret EA A-664.20889 -143.79889 or tonalite, and granite), whereas mid- and Late Cretaceous rocks have more ized heavy rare earth element (REE) contents. Sample 09AD-311, collected 1.5 09AD-343 17 95.8±1.5 mCret EA A-564.03307 -143.22725 08AD032A 18 93.9±1.3 mCret EA A-564.05306 -143.12472 93ADb37aT: 105.2±2.0 mCret EA A-264.08806 -141.65111 91ADb26cBt: 106.7±0.6 mCret EA A-664.23417 -143.63667 08AD054 mCret EA A-564.02472 -143.25944 09AD-311 mCret EA A-564.02605 -143.30186 restricted, quartz-rich compositions of granite and granodiorite (Fig. 9A). On a km north of the 109 Ma sample 13 (Fig. 3), is an altered biotite granite with 10ADb221970.0±1.2 LCretEA A-564.29611 -143.11194 91ADb29dBt: 69.1±0.19 LCretEA B-564.3314-143.3317 10ADb11ca. 70 LCretEA A-564.27861 -143.01417 10ADb16ca. 70 LCretEA A-564.31194 -143.14500 10ADb18ca. 70 LCretEA A-564.31333 -143.14167 10ADb30A ca. 70 LCretEA A-564.28500 -143.27333 K O-SiO diagram (Fig. 9B), most Late Triassic samples plot in the calc-alkaline smoky quartz that contains ubiquitous granophyric quartz-alkali feldspar in- 10ADb31A ca. 70 LCretEA A-564.31417 -143.36972 2 2 118A52168.1±0.8 LCretEA A-564.09184 -143.03780 81A5 22 67.9±1.1 LCretEA A-564.21326 -143.00528 129A52367.7±0.9 LCretEA A-564.11456 -143.01266 08ADb142467.0±1.5 LCretTA C-363.68194 -142.25806 08ADb132566.5±1.1 LCretTA C-363.65472 -142.29028 series and Early Jurassic samples span a range from calc-alkaline to high-K- tergrowths that have a herringbone-like texture (Figs. 7H, 7I) indicative of late 08ADb192669.9±1.1 LCretEA A-564.23611 -143.08278 08ADb032767.1±1.1 LCretEA A-464.21750 -142.90889 09AD-263 LCretEA A-564.24707 -143.13382 10AD-386 LCretEA A-564.05637 -143.06302 10AD-411 LCretEA A-564.10107 -143.02533 calc-alkaline to shoshonitic series; both age groups show a wide variability in eutectoid crystallization. 125A5-3LCret EA A-564.07229 -143.09734 09AD-292 LCretEA A-464.22994 -142.96938 140A528ca. 65 or youngerLCret EA A-564.02187 -143.09274 Mafic Rocks silica content (SiO range of ~52–72 wt%). Mid- and Late Cretaceous igneous Six Late Cretaceous samples from the ca. 70 Ma Middle Fork caldera 08ADb23E Mz EA A-564.0908-143.0336 08ADb21CretEA A-564.0106-143.3094 2 10ADb20LCret EA B-564.3186-143.1556

1 Data for all samples with a map number and those lacking a footnote are from this study. References: a—geochemical and age data published in Dusel- Bacon et al., 2009 (latitude and longitude for sample 93ADb01 supersedes the incorrect latitude and longitude listed for this sample in Dusel-Bacon et al. samples are more chemically restricted and plot within the high-K calc-alkaline have identical multielement patterns (Fig. 9J), similar to those of most of the 2009); b—age data previously published in Aleinikoff et al., 1981; c—age data previously published in Dusel-Bacon et al., 2002; d—age data previously published in Bacon and Lanphere, 1996; e—age for different sample from pluton published in Newberry et al., 1998a. Abbreviations: T, titanite U-Pb age; Z, zircon U-Pb age; Bt, biotite Ar-Ar age. 2 Age abbreviations: LTr, Late Triassic; EJur, Early Jurassic; mCret, mid-Cretaceous; LCret, Late Cretaceous; Mz, Mesozoic. 3 Quadrangle names refer to 1:63,360-scale quadrangles. Abbreviations: EA, Eagle; TA, Tanacross. series, with the exception of the 67 Ma syenite from Mount Fairplay, which mid-Cretaceous rocks. The ca. 68–66 Ma Late Cretaceous samples from the in- plots within the shoshonitic series. A plot of Al/(Na + K) versus Al/(Ca + Na + trusion north of Mount Veta, the felsic dikes and small intrusions associated K) (Fig. 9C) classifies most Late Triassic and Early Jurassic samples as metalu- with the Kechumstuk fault zone, and Mount Fairplay have multielement pat- 5Supplemental File 5. Location data and crystalliza- minous and most Cretaceous samples as either peraluminous or straddling terns similar to those from the mid-Cretaceous and ca. 70 Ma samples, but tion ages for whole-rock geochemistry samples of the boundary between these two fields; the 67 Ma syenite and quartz mon- show a wider range of trace element contents (Fig. 9K). Syenite from the felsic and mafic igneous rocks from the Fortymile district, east-central Alaska, and the map number for zonite samples from the Mount Fairplay intrusion plot in the metaluminous Mount Fairplay intrusion (24, Fig. 2) has the highest trace element contents and samples analyzed or dated in this study or the citation field. All of the samples follow a calc-alkalic compositional trend shown by the felsic dike sample (21, Fig 3) from within the Kechumstuk fault zone has the for previously published geochemical or geochrono- decreasing FeO*/MgO with decreasing TiO (Fig. 9D). In the Ta versus Yb tecton- lowest trace element contents. Microporphyritic aplite dike sample (28), the logical data. Please visit http://dx.doi.org/10.1130/ 2 GES01092.S5 or the full-text article on www.gsapubs. ic-discrimination diagram (Fig. 9E), almost all samples plot in the volcanic-arc U-Pb zircon age of which we interpret to be ca. 65 Ma or younger, plots below org to view Supplemental File 5. granite field. Late Cretaceous samples have slightly higher Ta concentrations all the ca. 68–66 Ma samples.

TABLE 3. WHOLE-ROCK ANALYSES OF IGNEOUS ROCKS FROM THE FORTYMILE DISTRICT, EAST CENTRAL ALASKA, PLOTTED IN FIGURES 9 AND 10

Map number, † Sample number source Age (Ma) Age SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2OK2OP2O5 LOI Ni Cr V Zr Ga Cu Zn La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Ba Th Nb YHfTaUPb Rb Cs Sr Sc

XRF (wt %, normalized to 100%) XRF (ppm) ICP-MS (ppm)

Felsic-Intermediate Rocks 09AD-319 1 215.5 ± 3.4LTr 55.30.72 19.96.580.152.798.334.461.430.331.157.4 12.1 130.891 22.4 4.0 79.1 18.9 36.54.4 17.93.8 1.33.3 0.53.0 0.6 1.6 0.2 1.40.2 541 2.15.7 14.92.3 0.20.6 5.5 29.60.7 998 12.3 08ADb24 2 215.0 ± 3.5LTr 63.80.61 17.54.430.131.695.663.961.960.240.530.3 3.9 99.4 113 19.5 13.9 62.1 15.5 28.83.5 13.9 3.11.0 3.0 0.5 2.80.6 1.6 0.21.5 0.2 897 2.7 10.6 15.32.9 0.6 1.4 10.8 54.61.3 645 8.2 09AD-338 4 207.9 ± 2.9LTr 69.50.27 17.30.830.020.663.495.971.83 0.110.351.8 6.6 35.394 18.93.9 11.3 7.1 13.71.7 6.61.4 0.51.2 0.2 0.90.2 0.50.1 0.50.1 616 1.57.3 5.12.2 0.5 1.2 5.9 34.20.4 908 3.2 93ADb01 a, bT: 212 nearby LTr 62.50.53 18.54.210.101.565.376.031.010.230.616.0 11.0 82.0 173 23.0 12.0 59.0 18.1 35.4 4.1 17.13.8 1.13.5 0.6 3.30.7 1.8 0.3 1.7 0.3 824 2.6 10.0 18.54.1 0.6 0.95.6 20.60.5 1197 8.0 (1.4 km to S) 93ADb10 aT: 215.7 ± 3.1LTr 65.70.42 17.63.100.081.184.764.752.190.170.490.0 4.0 63.097 17.0 8.0 45.0 12.6 22.5 2.5 9.92.1 0.71.8 0.31.5 0.3 0.8 0.1 0.8 0.1 844 2.7 10.18.3 2.6 0.7 0.86.5 53.10.8 745 6.7 LWM-07-06-124 31, a 210 ± 3LTr 62.30.58 18.94.090.101.765.844.931.270.251.570.0 6.6 78.9 106 20.1 1.7 60.0 19.4 32.93.8 14.52.9 1.0 2.4 0.32.0 0.4 1.00.1 0.90.1 715 3.39.8 9.72.7 0.71.3 5.7 23.04.7 1092 6.2 08ADb15 LTr 60.20.60 19.44.550.121.675.636.031.560.270.642.3 7.9 97.6 156 22.05.8 69.1 17.7 38.74.9 20.0 4.21.3 3.9 0.6 3.6 0.7 2.0 0.3 1.90.3 958 2.7 9.5 19.4 3.9 0.61.2 6.3 27.80.6 1293 8.1 10AD-394 LTr 66.30.38 17.52.850.091.173.485.482.660.142.373.9 9.2 57.8 102 18.25.3 54.5 10.5 18.4 2.3 8.81.8 0.61.6 0.21.4 0.30.8 0.10.7 0.1 1244 1.99.9 7.62.7 0.6 0.9 12.1 64.7 2.4 910 5.4 09AD-317 LTr 54.70.84 19.86.990.162.728.314.321.750.381.306.5 7.3 145.7 102 20.1 3.3 77.2 11.0 24.9 3.4 15.03.7 1.4 3.50.5 3.20.6 1.7 0.2 1.6 0.3 8521.0 7.6 16.72.3 0.40.6 5.8 29.60.6 1007 12.2 93ADb09 a, eT: 201.0 ± 1.4; Hbl: EJur 64.30.45 17.4 4.110.081.565.094.032.750.160.403.0 6.0 83.0 128 18.0 5.0 53.0 19.1 34.73.8 14.93.2 0.92.8 0.42.3 0.4 1.2 0.2 1.1 0.2 1208 5.2 8.6 12.43.4 0.51.1 9.8 80.61.9 820 8.6 197.3 ± 0.7 09AD-240 5 190.5 ± 4.8 EJur 71.40.21 15.21.860.050.742.653.664.130.120.964.9 12.8 35.8 86 15.2 2.4 43.3 10.8 21.32.5 9.3 1.90.5 1.60.2 1.4 0.3 0.7 0.1 0.80.1 1576 3.5 7.97.0 2.50.5 1.3 29.7 96.60.6 1248 6.2 08ADb22 7 186.2 ± 3.0 EJur 63.20.55 16.35.090.132.004.773.594.040.290.824.2 24.4 114.8 169 17.74.8 77.9 39.9 71.68.4 31.46.3 1.75.4 0.8 4.40.9 2.4 0.32.1 0.3 1851 12.8 10.6 23.3 4.20.8 1.711.9 79.50.7 1334 13.2 08AD100 8 184.8 ± 2.9 EJur 70.70.38 13.52.060.071.366.503.661.620.160.399.4 32.6 60.4 126 15.20.9 50.4 24.3 46.95.6 21.24.4 1.1 3.7 0.63.2 0.61.70.2 1.60.2 1979 7.99.6 17.33.4 0.71.8 8.8 18.80.2 1320 9.2 8A5-4 9 184.5 ± 3.0 EJur 57.90.61 16.67.030.162.435.213.735.860.490.454.9 8.6 174.6 145 17.2 47.7 93.7 37.5 69.28.3 32.06.7 1.95.4 0.8 4.10.8 2.00.3 1.8 0.3 1650 10.6 10.0 20.13.5 0.63.2 17.2 145.22.2 1396 16.2 10AD-368 10 183.4 ± 3.6 EJur 66.10.63 14.45.490.122.043.762.414.710.362.996.0 26.8 120.4 141 16.7 10.6 86.2 37.4 70.38.3 31.46.3 1.6 5.0 0.74.1 0.8 2.00.3 1.8 0.3 2097 10.9 11.7 20.13.7 0.8 3.7 14.9115.4 4.0 957 16.4 08ADb08 11 181.2 ± 2.6 EJur 56.31.06 13.59.060.215.478.383.202.130.631.17 30.4 143.0 281.0 194 18.9 18.5122.7 43.5 96.0 12.3 49.2 10.42.5 8.71.3 7.11.4 3.60.5 3.10.5 774 5.3 14.3 35.4 4.90.8 2.0 12.7 36.51.2 969 29.6 91ADb10 aZr: 188.4 ± 1.2; T: EJur 57.10.58 18.06.700.151.985.874.094.820.730.389.8 186 75.0 25.2 49.2 24.9 5.4 1.70.6 2564 2.79.0 29.03.7 0.4 1.3 99.00.7 1331 187.3 ± 0.6 91ADb35 aT: 184.6 ± 2.1 EJur 62.00.68 14.66.280.142.525.592.794.750.680.50 28.9 238 74.0 45.8 84.3 37.28.2 1.80.9 1574 11.4 11.0 37.06.2 0.9 1.8 111.00.3 996 93ADb18 aZ: 191.3 ± 0.4 EJur 72.20.17 15.41.810.040.463.102.524.210.061.744.0 5.0 17.0 121 17.06.0 42.0 20.5 35.93.8 14.4 3.01.1 2.5 0.3 1.6 0.3 0.60.1 0.6 0.1 1791 8.07.5 8.23.2 0.6 1.9 23.6 104.31.8 630 4.4 98ADb15 aT: 186.7 ± 1.6 EJur 66.70.60 15.04.66 0.111.964.662.373.670.321.085.0 20.0119.0 179 19.0 0.0 73.0 40.5 74.68.4 32.56.8 1.75.8 0.84.7 0.9 2.30.3 2.0 0.3 2043 12.7 12.0 23.74.5 0.82.3 13.9 92.42.2 715 15.2 98ADb30 aZ: 193.2 ± 0.4 EJur 52.10.80 16.68.860.194.989.923.472.620.490.81 22.0 64.0 209.0113 20.03.0 99.0 21.4 40.55.2 21.2 4.71.4 4.8 0.7 4.1 0.82.3 0.32.1 0.3 2199 2.27.0 22.21.8 0.40.7 5.8 34.80.3 1071 21.6 07ADb01A 29, aZ: 187 ± 3 EJur 63.90.60 14.02.970.103.927.252.893.970.351.92 17.5 99.3 133.3 137 15.0 5.6 58.2 35.6 67.6 7.9 28.75.7 1.44.8 0.74.00.8 2.10.3 1.90.3 1581 10.711.7 20.4 3.9 0.83.6 39.2 86.31.6 686 15.9 00ADb18 aZ: 197.6 ± 1.1 EJur 65.00.42 17.04.350.101.365.433.302.790.241.103.0 2.0 79.0 108 19.0 11.0 44.0 20.6 37.04.2 17.03.9 1.13.2 0.52.8 0.61.5 0.2 1.4 0.2 1730 4.8 10.4 15.82.7 0.62.7 8.0 70.0 1.7 1305 8.7 00ADb41 aZ: 197.2 ± 2.5 EJur 67.80.33 16.43.38 0.111.534.823.032.440.152.391.0 7.0 73.0 97 16.0 0.0 42.0 10.3 19.12.2 8.82.2 0.72.0 0.32.1 0.41.20.2 1.30.2 1785 3.87.0 12.02.5 0.62.1 13.5 74.03.8 850 9.2 08AD036 EJur 65.30.49 16.54.280.141.634.013.693.760.24 1.20 2.5 17.8 97.8 150 18.111.9 86.4 35.7 64.4 7.5 27.7 5.5 1.44.7 0.73.9 0.8 2.2 0.3 2.0 0.3 1630 12.1 10.3 20.84.0 0.83.4 17.1 75.8 0.91115 10.4 08AD039 EJur 55.61.16 13.29.890.235.838.842.701.800.701.02 31.8 157.1 304.0 221 19.6 34.6 132.8 47.7 106.4 13.8 55.411.62.7 9.91.4 7.91.5 4.0 0.6 3.40.5 620 6.4 15.8 38.75.5 1.02.0 11.7 27.10.9 885 30.6 08ADb18A EJur 58.60.88 14.48.360.183.746.632.594.010.560.916.2 41.6 212.0 210 20.0 23.1 123.0 56.7110.1 13.2 50.2 10.0 2.38.4 1.26.7 1.33.4 0.52.9 0.5 1319 13.6 14.2 32.85.5 1.02.3 17.2 82.8 3.7 885 26.4 09AD-229 EJur 67.70.30 17.12.700.051.084.515.231.170.140.859.7 13.3 54.2110 18.7 8.0 68.1 13.8 25.32.9 10.92.4 0.62.1 0.32.1 0.41.1 0.21.0 0.21071 3.2 4.8 10.62.8 0.21.0 15.3 20.30.4 1282 7.1 10AD-402 EJur 66.90.53 14.24.79 0.112.002.893.155.170.320.99 12.0 39.9 124.5 146 18.42.2 63.0 33.6 65.57.5 27.95.6 1.44.6 0.73.9 0.8 2.0 0.3 1.90.3 1925 9.6 10.4 20.1 4.0 0.72.7 15.0 140.71.2 996 13.8 08ADb02 EJur 71.00.23 15.52.190.080.893.753.352.890.102.780.0 7.9 44.278 14.80.0 36.28.2 15.21.8 7.11.6 0.51.5 0.3 1.60.3 1.00.1 1.0 0.2 2046 2.55.7 9.22.2 0.41.8 11.6 83.51.4 725 5.4 08ADb20 12 111.8 ± 1.5 mCret 77.90.04 12.30.770.090.040.313.784.710.010.320.0 5.33.6 62 14.3 17.2 16.111.4 28.74.1 17.2 6.70.1 8.1 1.8 12.42.7 8.4 1.4 9.6 1.673 28.5 19.2 81.3 3.73.5 4.1 39.7 275.92.1 24 5.6 08AD052 13 108.8 ± 1.7 mCret 66.90.58 15.84.580.101.603.942.993.250.200.830.0 5.1 72.6 157 16.92.8 68.1 39.3 76.6 8.9 32.96.2 1.35.1 0.84.4 0.92.3 0.32.1 0.3 1476 16.2 10.8 22.44.5 0.93.1 20.6 123.53.9 361 10.3 10ADb23 14 103.2 ± 1.5 mCret 74.40.16 14.01.640.060.311.543.983.800.040.680.0 2.8 12.4 125 16.40.8 42.8 35.0 65.6 8.1 29.26.5 1.06.1 1.1 6.81.4 4.0 0.63.9 0.6 1332 17.6 14.8 37.9 4.21.5 3.4 21.1 139.3 1.8 128 7.2 10ADb26A 15 101.4 ± 1.4 mCret 79.90.0111.7 0.110.000.000.432.964.910.010.432.0 2.43.4 38 13.0 0.0 3.5 12.4 26.13.4 12.53.7 0.24.0 0.85.3 1.23.4 0.53.5 0.6 192 13.5 9.1 35.02.3 1.23.2 20.5 157.7 0.6212.8 10ADb25 16 96.2 ± 1.3 mCret 70.70.36 15.42.830.060.713.443.283.100.144.030.3 3.4 29.5 156 19.50.6 49.5 36.2 69.38.5 31.35.8 1.34.3 0.63.2 0.6 1.4 0.2 1.20.2 1400 10.3 10.4 15.34.3 0.82.3 21.0 99.43.2 412 5.5 09AD-343 17 95.8 ± 1.5 mCret 73.80.21 14.01.970.060.411.443.324.710.060.742.9 5.0 15.5 132 13.02.3 28.8 34.1 62.6 6.6 21.8 3.90.8 3.1 0.5 2.90.6 1.7 0.3 1.8 0.3 1498 20.1 10.7 16.4 3.51.0 2.4 18.4 140.6 1.8 194 4.0 08AD032A 18 93.9 ± 1.3 mCret 76.40.25 13.71.630.060.461.383.232.810.071.560.4 5.2 18.9 143 13.6 31.2 33.6 35.8 64.56.8 22.54.0 0.73.2 0.53.0 0.6 1.70.3 1.70.3 1578 21.0 10.9 16.34.0 1.0 3.0 14.0 93.95.3 320 3.4 93ADb37 aT: 105.2 ± 2.0 mCret 67.20.42 16.63.820.121.144.443.452.600.200.612.0 0.0 49.0 144 17.08.0 67.0 30.1 54.86.3 24.65.4 1.4 4.8 0.84.5 0.92.6 0.4 2.60.4 1771 12.3 11.1 26.14.0 0.93.4 28.6 93.22.6 514 7.9 91ADb26 cBt: 106.7 ± 0.6 mCret 69.30.58 14.64.220.091.633.272.923.190.200.696.3 175 57.4 38.7 78.4 31.77.1 1.00.8 775 15.9 12.0 31.0 4.41.2 3.2 116.03.9 237 08AD054 mCret 77.70.05 12.70.850.030.060.283.704.610.020.530.0 2.42.4 75 14.1 0.0 19.9 12.8 25.9 3.4 12.53.4 0.2 3.60.7 4.91.0 3.00.5 3.4 0.5 241 29.6 15.5 28.9 3.42.3 2.9 34.8 180.91.5 44 2.9 09AD-311 mCret 77.10.10 13.11.290.030.170.233.314.640.030.881.1 5.95.8 87 13.3 1.9 25.0 22.6 44.04.8 16.73.7 0.4 3.10.6 3.50.7 2.00.3 2.2 0.4 505 23.611.9 19.5 3.11.5 2.2 32.0 166.52.9 77 3.1 10ADb22 19 70.0 ± 1.2 LCret 71.20.34 14.82.490.070.682.773.234.280.121.331.4 6.3 35.4 172 17.12.2 47.4 46.4 84.79.0 30.4 5.21.2 3.80.6 3.20.6 1.7 0.3 1.7 0.3 1478 20.4 15.3 17.14.3 1.1 6.9 25.8 147.4 5.1 550 5.3 91ADb29 dBt: 69.1 ± 0.19 LCret 71.10.36 15.12.570.070.972.542.954.240.122.225 7.6 39.1 173.1163.3 56.5 54.2 94.29.8 32.95.2 1.23.7 0.53.0 0.6 1.50.2 1.50.3 1621 19.6 13.9 16.1 4.51.0 5.0 24.1 146.55.9 515 5.1 10ADb11 ca. 70 LCret 71.60.32 15.02.450.080.702.353.184.21 0.112.230.3 4.8 30.4 175 16.8 1.3 46.8 54.7 97.3 10.1 33.25.3 1.23.7 0.53.1 0.6 1.6 0.21.6 0.3 1547 21.1 14.4 16.14.5 1.16.2 25.2 141.22.7 546 4.5 10ADb16 ca. 70 LCret 71.30.33 15.02.340.080.622.913.064.30 0.111.741.2 5.1 31.8 171 17.32.4 44.3 50.2 90.1 9.4 31.8 5.31.2 3.7 0.6 3.10.6 1.6 0.21.60.3 1504 20.7 14.9 16.64.4 1.17.6 21.4 149.1 5.5 532 4.9 10ADb18 ca. 70 LCret 69.20.42 15.43.120.071.063.103.194.180.171.870.0 3.4 45.9 199 16.9 4.1 46.0 56.0 99.2 10.5 35.5 5.81.3 4.1 0.6 3.30.6 1.70.3 1.6 0.3 1864 19.1 14.2 17.34.8 1.04.9 19.4 135.4 2.1 665 6.2 10ADb30A ca. 70 LCret 72.30.29 14.62.120.070.642.043.304.520.101.770.3 3.8 29.9 144 16.41.0 39.9 37.9 66.87.7 26.44.9 1.0 3.9 0.63.5 0.71.8 0.3 1.8 0.3 1247 20.7 18.3 18.54.0 1.56.4 24.3 169.42.3 465 5.0 10ADb31A ca. 70 LCret 71.00.34 15.12.620.080.762.423.454.140.121.212.4 5.5 34.9 181 15.6 2.0 44.5 50.7 89.79.5 31.75.3 1.2 3.80.5 3.1 0.61.7 0.31.6 0.3 1482 20.7 14.8 16.64.7 1.1 5.8 21.7 139.94.3 618 5.4 118A5 21 68.1 ± 0.8 LCret 77.80.15 13.81.030.030.400.122.494.160.041.612.7 9.5 12.5 100 19.09.3 96.8 17.2 32.33.5 12.5 3.00.3 2.8 0.5 2.9 0.6 1.50.2 1.5 0.2 725 21.6 18.3 16.0 3.92.6 4.9 42.6 199.74.8 86 2.9 81A5 22 67.9 ± 1.1 LCret 71.10.42 14.72.670.050.922.553.154.320.120.61 3.9 11.6 38.4 195 19.2 0.3 31.9 50.1 91.2 10.1 34.3 5.71.1 3.9 0.5 3.00.5 1.4 0.2 1.30.2 1590 18.2 14.4 14.7 5.1 1.23.8 17.1 146.52.5 561 6.6 129A5 23 67.7 ± 0.9 LCret 71.70.36 14.92.340.030.732.503.264.100.100.682.1 9.3 27.7 181 18.32.5 42.1 41.6 77.78.4 28.5 4.71.0 3.3 0.5 2.40.4 1.10.2 1.1 0.2 1737 18.0 10.3 12.0 4.9 1.13.1 17.5 126.61.7 592 5.4 08ADb14 24 67.0 ± 1.5 LCret 56.40.75 19.75.530.131.645.223.816.340.471.072.2 6.9 75.2 339 20.6 67.9 97.2 85.6 151.7 16.2 55.39.4 2.67.0 0.95.0 1.02.5 0.4 2.10.3 4493 47.4 34.7 25.37.5 2.0 13.7 66.9 240.9 19.1 1400 6.7 08ADb13 25 66.5 ± 1.1 LCret 68.60.49 15.02.970.061.392.553.625.150.120.579.5 45.8 52.8 266 18.7 19.9 55.1 54.4 93.6 9.6 30.55.1 1.03.9 0.6 3.40.7 1.80.3 1.80.3 1312 50.7 22.4 18.17.1 2.0 7.7 38.6 281.8 24.3 398 5.9 08ADb19 26 65.8 ± 1.5 LCret 69.40.43 14.92.950.041.433.132.954.580.170.450.3 20.5 61.5 134 18.00.0 30.5 39.2 71.17.6 25.9 4.6 1.03.7 0.63.4 0.71.9 0.3 1.9 0.3 970 18.4 20.4 18.64.2 2.98.1 15.1 191.95.4 441 7.9 08ADb03 27 65.8 ± 1.4 LCret 72.10.27 14.81.850.030.592.123.244.910.080.510.0 8.8 25.4117 15.80.9 34.0 34.4 63.77.3 25.25.0 0.94.0 0.63.6 0.7 1.90.3 1.80.3 1359 20.0 13.1 19.03.9 1.7 3.3 21.7 184.43.2 412 4.8 09AD-263 LCret 69.60.41 15.62.630.040.992.313.454.850.161.393.4 7.0 41.7 186 16.85.4 63.7 52.8 94.0 10.0 34.35.8 1.4 4.30.6 3.60.7 1.8 0.3 1.7 0.3 2318 19.8 15.4 18.44.7 1.1 5.5 36.6 149.1 2.1 771 6.4 10AD-386 LCret 71.60.32 14.82.180.040.763.052.704.400.103.732.3 8.4 26.3 146 18.4 0.7 23.0 36.6 66.57.2 24.24.0 0.92.8 0.42.2 0.41.2 0.2 1.10.2 1512 17.4 10.511.94.0 1.03.2 11.0 152.7 6.7 495 4.6 10AD-411 LCret 75.90.21 14.31.320.010.380.102.934.750.061.724.7 10.9 17.5 125 21.66.9 65.4 21.8 36.84.1 14.02.8 0.42.3 0.42.0 0.41.1 0.2 1.1 0.2 842 17.7 15.211.03.9 1.93.3 60.8 218.66.2 109 3.0 125A5-3 LCret 71.40.36 14.92.370.060.842.283.254.360.121.295.1 8.7 32.2 154 18.35.8 46.1 43.4 78.8 8.4 28.0 4.5 1.03.1 0.42.4 0.5 1.2 0.2 1.2 0.2 1701 19.411.3 12.5 4.2 1.05.0 21.8 145.3 3.0 657 5.1 09AD-292 LCret 71.80.33 15.22.160.030.722.223.623.820.100.914.3 10.5 27.1 152 18.8 0.7 25.3 34.8 64.4 7.0 24.44.4 0.93.2 0.52.6 0.51.3 0.21.2 0.2 1419 14.011.3 13.3 4.01.0 2.411.8 136.52.6 568 5.3 140A5 28 ca. 65 or younger LCret 74.00.15 14.61.100.030.361.134.763.890.030.681.8 11.7 26.1 81 17.3 1.4 25.4 10.9 19.32.2 8.0 1.60.4 1.40.2 1.40.3 0.90.1 1.00.2 1742 5.08.1 9.12.4 0.5 1.7 31.3 88.21.0 1193 3.7 Maﬁc Rocks 08ADb23E Mz 52.31.06 17.77.860.285.85 10.23 3.79 0.63 0.29 1.04 43.0 143.0 306.5110 17.8 158.9 536.5 16.3 34.44.5 18.8 4.3 1.44.3 0.74.2 0.82.3 0.32.10.3 455 2.14.5 21.8 3.10.3 0.9112 18.20.5 856 30.4 08ADb21 Cret 50.70.97 17.7 10.11 0.21 6.50 10.65 2.00 0.97 0.20 1.30 5.0 39.8 256.177 15.511.4 84.7 15.7 32.24.0 16.43.9 1.13.9 0.63.9 0.82.1 0.3 1.80.3 431 4.35.3 19.72.1 0.3 0.7 8.4 33.42.3 419 34.2 10ADb20 LCret 53.91.25 17.79.200.264.516.173.633.060.261.962.3 8.0 246.0 120 22.7 25.9 109.3 43.4 83.3 10.2 38.77.7 2.26.5 1.05.7 1.13.0 0.4 2.70.4 1353 10.411.1 29.53.6 0.6 2.0 14.9116.1 12.4 897 27.5 Note: Sample location data given in Supplemental File 1 (see footnote 1). Abbrevations: T—titanite U-Pb age; Z—zircon U-Pb age; Bt—biotite Ar-Ar age; Hbl—hornblende Ar-Ar age; LTr—Late Triassic; EJur—Early Jurassic; mCret—mid-Cretaceous; LCret—Late Cretaceous; Mz—Mesozoic; LOI—loss on ignition; XRF—X-ray diffraction; ICP-MS—inductively coupled plasma-mass spectrometry. *Map identiﬁcations for U-Pb dated samples are shown in Figures 2 or 3. Data for all samples with map number and those lacking a footnote are from this study. †References: a—geochemical and age data from in Dusel-Bacon et al. (2009); b—from Aleinikoff et al. (1981); c—from Dusel-Bacon et al. (2002); d—from Bacon and Lanphere (1996); e—denotes age for different sample from pluton published in Newberry et al., 1998a.

vol% Q A 7 EXPLANATION for Figures A–K Quartzolite (silexite) B Shos: Basalt Shos. Latite Trachyte 6 Rest: Basalt BasAnd Andesite Dacite Rhyolite Tertiary (<65 Ma) 5 O

Quartz-rich 2 Late Cretaceous (68–66 Ma) Shoshonitic Series granitoids K 4

Late Cretaceous (70 Ma) % High-K Calc-Alkaline Series Mid-Cretaceous (96–94 Ma) 3 Wt. Mid-Cretaceous (112–101 Ma) 2 e Calc-Alkaline Series Early Jurassic (199–181 Ma) Granite 1 T Arc Tholeiite Series Late Triassic (216–208 Ma) onalite 0 45 50 55 60 65 70 75 Granodiorite Alkali-feldspar granit Wt. % SiO2

Alkali-feldspar Quartz Quartz diorite/ quartz syenite Quartz Quartz monzodiorite/ quartz gabbro/ syenite monzonite quartz quartz anorthosite monzogabbro Alkali-feldspar Monzonite syenite Syenite Diorite/ gabbro/ anorthosite vol% Or Monzodiorite/ vol% Ab+An monzogabbro

3 4 100 C E Metaluminous Peraluminous D 3 WPG 10 2 syn-COLG 2

O 2 Thol Ta Ti 1

Al/(Na+K) 1 Peralkaline 1 ORG CA VAG .1 0 0 0.5 1.0 1.5 2.0 01234.1 110 100 Al/(Ca+Na+K) FeO*/MgO Yb

Figure 9. Selected discrimination diagrams and primitive mantle-normalized multielement plots of whole-rock geochemical data for felsic and intermediate composition Mesozoic igneous rocks

from the Fortymile district. (A) International Union of Geological Sciences plutonic rock classification of Streckeisen (1976). Or—orthoclase; Ab—albite; An—anorthite; Q—quartz. (B) K2O-SiO2 di-

agram of Peccerillo and Taylor (1976) showing fields for various series and compositions of arc volcanics. (C) Shand’s index classification (Maniar and Piccoli, 1989). (D) Variation trend of TiO2 with increasing FeO* (all iron calculated as FeO)/MgO. Thol—example of a tholeiitic compositional trend; CA—example of a calc-alkaline compositional trend (Miyashiro, 1974). (E) Ta-Yb tectonic discrimination diagram (Pearce et al., 1984). WPG—within-plate granite; syn-COLG—syncollisional granite; VAG—volcanic-arc granite; ORG—ocean ridge–type granite. (Continued on following page).

1000 F Late Triassic (216–208 Ma) G Early Jurassic (199–181 Ma)

100

Rock/Primitive Mantle 1

.1 Th Nb Ta La Ce Pr NdSm Zr Hf Eu Ti Gd Tb Dy YErYbLu Th Nb Ta La Ce Pr NdSm Zr Hf Eu Ti Gd Tb Dy YErYbLu

1000 H Mid-Cretaceous (112–101 Ma) I Mid-Cretaceous (96–94 Ma)

100 Figure 9 (continued ). (F–K) Primitive mantle-normalized plots of dated samples from the six Mesozoic age groups. Black asterisk is Tertiary sample. Primitive mantle values are those 10 of Sun and McDonough (1989); com- patibility of elements, i.e., the degree to which they favor the solid phase during melting or crystallization, in-

Rock/Primitive Mantle 1 creases to the right. Geochemical data are given in Table 3.

.1 ThNbTa LaCe Pr NdSmZr Hf Eu Ti GdTbDy YErYbLu Th Nb Ta La Ce Pr NdSm Zr Hf Eu Ti Gd Tb Dy YErYbLu

1000 J Late-Cretaceous (70 Ma) K Late Cretaceous (68–66 Ma) and Tertiary <65 Ma 100

Rock/Primitive Mantle 1

.1 Th Nb Ta La Ce Pr NdSm Zr Hf Eu Ti Gd Tb Dy YErYbLu Th Nb Ta La Ce Pr NdSm Zr Hf Eu Ti Gd Tb Dy YErYbLu

Mafic Rocks gram (Fig. 10C); and (3) have highly developed Nb-Ta troughs and negatively sloping multielement patterns (Fig. 10D). Thus, tectonic signatures based on Three samples of mafic igneous rocks were collected for whole-rock geo- trace element concentrations of the mafic rocks are consistent with those in- chemical analyses in order to compare their trace element signatures with dicated for the Cretaceous felsic and intermediate-composition igneous rocks. those of the felsic and intermediate-composition rocks. Interpretation of mafic geochemistry is more straightforward than that involving felsic rocks because: (1) felsic magma not derived in an arc setting can acquire an arc-like geochem- Pb Isotope Data ical signature as a result of generation in, or contamination by, continental crust; (2) felsic magmas can represent blending of partial melt contributions In Table 4 we present 15 new analyses of Pb isotopic compositions of feld- from many different continental lithologies (e.g., Piercey et al., 2001); and (3) spar separates from dated igneous rocks in the western Fortymile district, as the abundances of some high field strength elements (HFSEs) (including Ti, Zr, well as an analysis of feldspar from the Late Triassic granodiorite of the Taylor and Hf) and REEs are extremely sensitive to accessory mineral fractionation Mountain batholith, east of the study area. We also report new and previously and removal from the melt. published Pb isotopic compositions of sulfides from the LWM, Eva, Drumstick, The first mafic igneous rock sample is Mesozoic diorite, collected ~200 m and Oscar Pb-Zn-Ag-Cu prospects in the western Fortymile district. Sample south of the dated Late Triassic hornblende-biotite granodiorite from the pluton locations are shown in Figure 11, with the exception of that for feldspar from of Kechumstuk Mountain (2, Fig. 3) near the Mitchell prospect; it is weakly foli- Taylor Mountain, which is shown in Figure 2. ated, fine grained, and composed of hornblende and plagioclase. The relation- Pb isotopic compositions of feldspars from igneous rocks provide a means ship of the diorite to the Triassic intrusion is unknown. The presence of foliation of identifying the contributions of mantle and crustal Pb in parental magmas in the diorite suggests a pre-Cretaceous age, and we interpret the diorite to be and characterizing magmatism both spatially and temporally. Figure 12A either a cognate phase of the Late Triassic intrusion or a Jurassic dike related to shows the Pb isotopic ratios of the 15 igneous feldspars determined in our the Mount Veta intrusion. The second sample is a nonfoliated diorite dike that study. Figure 12B presents the isotopic compositions of 30 previously pub- cuts the ca. 110 Ma Corner granite. The diorite is medium grained and contains lished analyses of igneous feldspars collected in east-central Alaska (Aleinikoff approximately equal amounts of hornblende and highly altered plagioclase. The et al., 1987, 2000; Dilworth et al., 2007) in relation to the fields defined by the ig- absence of a preferred fabric in the dike suggests a mid-Cretaceous or younger neous feldspar compositions determined in our study. The feldspar Pb isotopic age; because Tertiary dikes, where mapped in the adjacent Big Delta quadrangle compositions are divided into the following age groups, based on the U-Pb zir- to the west (Day et al., 2007), are generally unaltered, we consider a Tertiary age con or titanite crystallization age of the sample: Late Triassic to earliest Juras- less likely. The third sample is a rare magmatic enclave within the ca. 70 Ma sic (pluton of Diamond Mountain) (215–199 Ma), Early Jurassic (187–181 Ma), granite porphyry of the Middle Fork caldera. The 6 × 8 × 12 cm mafic enclave is mid-Cretaceous (115–95 Ma) and Late Cretaceous (94–78, 73–70, and 68–66 Ma) a fine-grained hornblende-plagioclase-biotite diorite with a texture that shows (Fig. 12.) All of the samples in the 94–78 Ma age group and all but 2 of the 18 that the enclave magma crystallized rapidly in undercooled conditions. Bacon samples in the 115–95 Ma age group (Fig. 12B) are from outside the study et al. (2014) suggested that the enclave may be similar to parental magma of the area, in other parts of the Yukon-Tanana Upland or the foothills to the Alaska caldera and indicates an input of higher temperature mafic magma. Range south of the Tanana River (Aleinikoff et al., 2000; Dilworth et al., 2007). Whole-rock geochemical analyses of the mafic rocks plot in the calc-alkalic The shale curve of Godwin and Sinclair (1982), which closely approximates the arc field on immobile trace element diagrams utilizing HFSEs (Figs. 10A, 10B). Pb isotopic evolution of upper crustal rocks in the northern Cordillera (includ- A plot of Nb/Yb versus Th/Yb (Fig. 10C) has been shown to be relatively un- ing both the parautochthonous YTa and the allochthonous YTT; Mortensen et affected by partial melting and fractional crystallization and to reflect mantle al., 2006), the average crustal growth curve of Stacey and Kramers (1975), and sources of basalt (Pearce, 1983; Pearce and Peate, 1995). Global averages or the estimated average mantle curve of Doe and Zartman (1979) are shown for typical values for characteristic magma settings, as well as trends for with- reference in Figure 12, as is the field of feldspar data derived from Pb isotopic in-plate enrichment, crustal contamination, and subduction-zone enrichment compositions of Late Devonian and Early Mississippian metaigneous rocks of (slab metasomatism), are shown for comparative purposes in Figure 10C. Mul- the parautochthonous YTa (Aleinikoff et al., 1987). tielement patterns (Fig. 10D) exhibit negative Nb and Ta anomalies relative to Feldspar from rocks of Late Triassic–earliest Jurassic, Early Jurassic, 94–78 Th and La, a geochemical characteristic that is typical of modern arcs. Ma early-Late Cretaceous, and others of Late Cretaceous age, in our study and The Cretaceous dike and especially the mafic enclave show geochemical other studies have 206Pb/204Pb ratios that range from 18.724 to 19.454 and define evidence of crustal contamination in elevated Th contents, which cause them to fields that become more radiogenic with time. On 206Pb/204Pb versus 207Pb/204Pb (1) plot very close to the composition of average upper crust (Fig. 10A) within diagrams (Figs. 12A, 12B), these four age groups plot below the shale curve of the calc-alkalic end of the volcanic arc basalt field; (2) follow the trend of crustal Godwin and Sinclair (1982) and are significantly more radiogenic than values contamination or subduction zone enrichment in the Nb/Yb versus Th/Yb dia- for the average crustal growth curve of Stacey and Kramers (1975). In contrast,

Hf/3 EXPLANATION Y/15 ABs for A–C arc NMORB Mzd tholeiite Kd LKd transitional BABB NMORB Primitive arc tholeiite

EMORB EMORB Arc basalt

Calc-alkalicUCC OIB (rift) Alkalic Calc-alkalic Cont. arc

Th Ta La/10 Nb/8

10 1000 Subduction Zone Enrichment C Crustal Contamination D Within-Plate Enrichment OIB LKd (10ADb20) 100 Mzd (08ADb23E) 1 CAB Kd (08ADb21)

EMORB Th/Y b

IAT 10 0.1 Rock/Primitive Mantle array of basalts from NMORB non-subduction settings

1 ThNbTa LaCe Pr NdSmZr Hf Eu Ti GdTbDy YErYbLu 0.01 0.1 1 10 100 Nb/Yb

Figure 10. Immobile trace element discrimination diagrams and primitive mantle-normalized multielement plots of whole-rock geochemical data for Mesozoic diorite from the Fortymile district. Mzd—Mesozoic diorite; Kd—Cretaceous(?) diorite; LKd—Late Cretaceous fine-grained diorite enclave. Geochemical data and sample information are given in Table 3 and Supple- mental File 5 (see footnote 5). (A) Th-Hf-Ta diagram of Wood (1980). UCC is the composition of average upper continental crust (values of McLennan, 2001). EMORB—enriched mid-ocean ridge basalt; NMORB—normal mid-ocean ridge basalt; OIB—ocean-island basalt. (B) La-Y-Nb diagram of Cabanis and Lecolle (1989). BABB—backarc basin basalt; cont.—continental. (C) Nb/Yb vs Th/Yb ratio plots from Pearce (1983) and Pearce and Peate (1995). CAB—calc-alkalic basalt; IAT—island arc tholeiite. (D) Primitive mantle-normalized multielement plots. Primitive mantle values are those of Sun and McDonough (1989).

TABLE 4. Pb ISOTOPIC COMPOSITIONS OF IGNEOUS FELDSPAR AND SULFIDE SAMPLES FROM THE FORTYMILE DISTRICT, EAST-CENTRAL ALASKA

Sample Map Mineral U-Pb age* Latitude Longitude Reference for Pb Rock type, location or prospect name 206Pb/204Pb Error 207Pb/ 204Pb Error 208Pb/204Pb Error 207Pb/206Pb Error 208Pb/206Pb Error identiﬁcation (Ma) (decimal °) (decimal °) isotope

Feldspar 93ADb10 Fs 215.7 ± 3.11 64.03833 –142.82722 This study Hbl Bt granodiorite, Kechumstuk Mountain 18.7232 0.006 15.609 0.005 38.397 0.014 0.8337 0.0001 2.0508 0.0002 08ADb15 (resampling 3Fs 212.0 ± 3.32 63.9906 –142.1286 This study Hbl Bt Qtz monzodiorite, Taylor Mountain 18.751 0.012 15.657 0.010 38.458 0.026 0.8350 0.0002 2.0510 0.0005 of 79AFr2005) LWM-07-06-124 150 m west Pl 210 ± 33 64.23388 –142.82526 This study Hbl Qtz diorite, LWM prospect 18.724 0.005 15.607 0.004 38.369 0.011 0.8335 0.0001 2.0492 0.0001 of 6 93ADb09 Fs 201.0 ± 1.41 64.14222 –142.68278 This study Hbl Bt granodiorite, Diamond Mountain 18.870 0.002 15.660 0.001 38.683 0.004 0.8299 0.0000 2.0500 0.0001 08ADb08 11 Fs 181.2 ± 2.6 64.21444 –142.88417 This study Kfs megacrystic Hbl Qtz monzonite, near 19.075 0.005 15.655 0.004 38.893 0.010 0.8207 0.0000 2.0389 0.0001 West Little Whiteman prospect 07ADb01A 29 Kfs 187 ± 33 64.25680 –142.70002 This study Hbl Qtz monzonite, near Fish prospect 19.142 0.013 15.682 0.007 39.043 0.028 0.8192 0.0004 2.0397 0.0005 08ADb22 7 Kfs 186.2 ± 3.0 64.09306 –143.14583 This study Hbl Qtz monzonite, Mount Veta 19.100 0.015 15.664 0.007 38.888 0.038 0.8201 0.0005 2.0360 0.0012 8A5-4 9 Kfs 184.5 ± 3.0 64.0599 –143.1701 This study Hbl Cpx monzonite, Mount Veta 19.093 0.013 15.643 0.007 38.749 0.033 0.8193 0.0004 2.0295 0.0010 08ADb20 12 Kfs 111.8 ± 1.5 64.00889 –143.30972 This study Leucogranite, Corner pluton 19.454 0.002 15.736 0.002 39.444 0.005 0.8089 0.0000 2.0276 0.0002 08AD-052 13 Pl 108.8±1.7 64.02861 –143.24639 This study Altered Bt granite, Corner pluton 19.342 0.000 15.718 0.000 39.248 0.001 0.8126 0.0000 2.0291 0.0000 07-M-142 Kfs 71.6 ± 0.64 63.64219 –141.45731 This study Granodiorite, Bluff prospect 19.352 0.003 15.668 0.002 39.175 0.008 0.8096 0.0001 2.0243 0.0002 10ADb22 19 Fs 70.0 ± 1.25 64.2963 –143.1122 This study Rhyolite tuff, Middle Fork caldera 19.441 0.002 15.686 0.002 39.268 0.005 0.8068 0.0000 2.0198 0.0001 10ADb17 20 Fs 69.7 ± 1.25 64.3119 –143.1442 This study Granite porphyry, Middle Fork caldera 19.398 0.001 15.667 0.001 39.179 0.002 0.8077 0.0000 2.0198 0.0000 08ADb19 26 Kfs 65.8 ± 1.5 64.23611 –143.08278 This study Bt Hbl granite, near Oscar prospect 19.340 0.008 15.703 0.004 39.212 0.016 0.8119 0.0002 2.0276 0.0003 08ADb03 27 Kfs 65.8 ± 1.4 64.21750 –142.90889 This study Bt granite, near West Little Whiteman 19.429 0.011 15.669 0.005 39.251 0.033 0.8065 0.0004 2.0202 0.0012 prospect 129A5 23 Fs 67.7 ± 0.7 64.11456 –143.01266 This study Bt granite, Kechumstuk fault zone 19.499 0.007 15.765 0.005 39.535 0.015 0.8085 0.0001 2.0275 0.0002

Sulﬁde FMM1aGl 64.23378 –142.82404 Dusel-Bacon et al. Little Whiteman prospect 19.443 0.018 15.693 0.021 39.353 0.071 0.8071 0.0004 2.0241 0.0018 (2009) LWM-06-01-37 a Sl 64.23378 –142.82404 Dusel-Bacon et al. Little Whiteman prospect 19.521 0.034 15.681 0.031 39.210 0.092 0.8033 0.0004 2.0086 0.0019 (2009) LWM-08-04-96 b Gl 64.23506 –142.81820 this study Little Whiteman prospect 19.434 0.006 15.693 0.003 39.310 0.013 0.8075 0.0002 2.0227 0.0002 FMM2 c Gl ~64.105467 ~–143.196683 Dusel-Bacon et al. Eva prospect 19.317 0.018 15.692 0.021 39.214 0.071 0.8123 0.0004 2.0300 0.0018 (2009) 07ADb03B c Gl 64.10547 –143.19668 Dusel-Bacon et al. Eva prospect 19.329 0.009 15.698 0.013 39.204 0.048 0.8128 0.0013 2.0278 0.0008 (2009) JS-08-1B d Gl 64.12900 –143.20849 This study Drumstick prospect 19.337 0.003 15.712 0.001 39.306 0.006 0.8125 0.0001 2.0326 0.0001 JS-08-1A e Gl 64.12958 –143.20885 This study Drumstick prospect 19.347 0.006 15.725 0.003 39.357 0.015 0.8128 0.0002 2.0342 0.0005 JS-08-8D f Gl 64.23333 –143.09444 This study Oscar prospect 19.338 0.002 15.682 0.001 39.252 0.005 0.8110 0.0001 2.0298 0.0002 JS-08-8F f Gl 64.23333 –143.09444 This study Oscar prospect 19.317 0.010 15.681 0.010 39.240 0.010 0.8118 0.0020 2.0314 0.0030 07ADb02A g Py 64.23380 –143.09017 Dusel-Bacon et al. Oscar prospect 19.301 0.008 15.645 0.013 39.106 0.047 0.8112 0.0013 2.0256 0.0008 (2009)

Note: Analyses made by J.E. Gabites at the Pacific Centre for Isotopic and Geochemical Research, University of British Columbia, Canada. Pb isotopic compositions for sulfide minerals were determined using the methodology described in Mortensen et al. (2008). Pb isotopic ratios were corrected for mass fractionation of 0.10–0.12%/amu, based on replicate analyses of the NBS-981 common Pb standard and the composition recommended by Thirlwall (2000). Feldspar Pb isotopic compositions were measured on strongly acid-leached mineral separates, followed by HF dissolution and the same ion exchange methods that were used for trace sulfides. Mineral abbreviations: Hbl—hornblende; Bt—biotite; Qtz—quartz; Cpx—clinopyroxene. Minerals analyzed for lead isotopes: Fs—mixed intergrown K-feldspar and plagioclase; Kfs—K-feldspar, Pl—plagioclase; Gl—galena; Py—pyrite; Sl—sphalerite. Sample number and lowercase letter locations are shown in Figure 11. Map identifications for samples from which feldspars were separated are referred to in the text and shown in Figure 3 (Fig. 2 for 3) and in Tables 2 and 3. Errors are 2σ absolute values. *U-Pb zircon crystallization age determined in this study, unless otherwise indicated. Superscripts: 1— thermal ionization mass spectrometry titanite age from Dusel-Bacon et al. (2009; location of sample 93ADb09 was incorrectly plotted in Fig. 4 therein, but it is correctly shown in Fig. 11 of this paper). 2—This Pb isotope sample was a resampling of the locality from which zircon sample 79AFr2005 (map identification number 3 in Fig. 2 and Table 1) was collected. 3—SHRIMP (sensitive high-resolution ion microprobe) zircon age from Dusel-Bacon et al. (2009). 4—Laser ablation–inductively coupled plasma-mass spectrometry zircon age from Allan et al. (2013). 5—SHRIMP zircon ages for samples 10ADb17 and 10ADb22 from Bacon et al. (2014).

143°20'W 143°10'W 143°0'W 142°50'W 142°40'W

64°20'N

64°20'N 20

Sheba Fish x 29 4815 Mountain 26 g b f Oscar 6 WLWM a LWM 27 11

5825Mount Veta Figure 11. Locations of Pb isotope samples, western Fortymile district, east-central Alaska. Pb isotope compositions are giv- 64°10'N en in Table 4. Base is the U.S. Geological 64°10'N Survey 1:250,000-scale topographic map of the Eagle quadrangle, Alaska, published in 1957 with limited revisions in 1982. 93ADb09 e 5107 Drumstick x d Diamond 23 Mountian c Eva 7 Mitchell

93ADb10 13

024 12 Kilometers 1:250,000 5002 Kechumstuk 64°0'N Mountain 64°0'N

143°20'W 143°10'W 143°0'W 142°50'W 142°40'W

23 Feldspar Pb isotope—Number corresponds with Map Number of zircon U-Pb sample (Figs. 2 and 3; Table 2) d Sul de Pb isotope LWM Pb-Zn-Ag-Cu prospect

A C Feldspar in igneous rocks, this study Sulfides, this study 15.85 15.85 Cretaceous Cretaceous Devonian–Mississippian Devonian–Mississippian metaigneous rocks metaigneous rocks

Shale curve 15.75 15.75 (Godwin and Sinclair,1982) 0 0

Pb 200 Pb 200 400 400 XX 204 + B 204 X / 600 ++ / 600 15.65 Feldspar in plutonic rocks, east–central Alaska 15.65 Pb 800 0 Pb 800 15.85 0 187–181 Ma Cretaceous 187–181 Ma 207 200 207 200 215–199 Ma Devonian–Mississippian 215–199 Ma 400 metaigneous rocks 400 15.55 600 Cretaceous 15.55 600 average crustal curve rocks X LWM Gl & Sl (Stacey and Kramers, 1975) 15.75 68–66 Ma Eva Gl 0 0 mantle curve + 73–70 Ma Pb 200 0 Drumstick Gl 140 140 (Doe and Zartman, 1979) 115–95 Ma 400 Oscar GL & Py 15.45 204 + 15.45 / 600 0 15.65 0

Pb 0 + 39.8 Devonian–Mississippian 800 + 39.8 Devonian–Mississippian metaigneous rocks 187–181 Ma metaigneous rocks 207 200 Cretaceous 39.4 200 400 39.4 200 215–199 Ma rocks X + 15.55 600 X ++ 68–66 Ma X 39.0 Cretaceous + 73–70 Ma 39.0 Shale curve 400 400 Cretaceous 94–78 Ma (Godwin and Sinclair,1982) 0 Pb 140 Pb 0 115–95 Ma 0 38.6 187–181 Ma 15.45 38.6 187–181 Ma 204 204

/ 600 17.6 18.0 18.4 18.8 19.2 19.6 / 600 215–199 Ma 206 204 215–199 Ma Pb 38.2 200 Pb/ Pb Pb 38.2 200

208 800 average crustal curve 208 800 37.8 400 (Stacey and Kramers, 1975) 37.8 400 Cretaceous 0 rocks 0 X LWM Gl & Sl 68–66 Ma Eva Gl 37.4 140 37.4 140 mantle curve + 73–70 Ma Drumstick Gl (Doe and Zartman, 1979) 115–95 Ma Oscar GL & Py 37.0 37.0 17.6 18.0 18.4 18.8 19.2 19.6 17.6 18.0 18.4 18.8 19.2 19.6 206Pb/204Pb 206Pb/204Pb

Figure 12. 206Pb/204Pb versus 207Pb/204Pb and 206Pb/204Pb versus208Pb/204Pb plots for feldspars from igneous rocks. (A) From our study; data for feldspar Pb are given in Table 4. (B) From plutonic rocks in east-central Alaska; data for feldspar Pb are from Aleinikoff et al. (1987, 2000) and Dilworth et al. (2007). (C) From sulfides from base metal prospects in the Fortymile mining district; data for sulfide Pb are from Dusel-Bacon et al. (2009) and Table 4. LWM—Little Whiteman prospect; Gl—galena; Py—pyrite; Sl—sphalerite. Colored fields shown in B and C show the age-based fields determined for feldspars from our study. Analytical uncertainties are generally smaller than the plot symbols.

feldspar Pb isotopes from 115–95 Ma mid-Cretaceous rocks from our study (n rates: ≥900 °C, U-Pb closure temperature of zircon (Heaman and Parrish, 1991): = 2) plot above the shale curve and within the range of previously analyzed 660–700 °C, U-Pb closure temperature of titanite (Scott and St-Onge, 1995); mid-Cretaceous granitic rocks of this age from adjacent parts of east-central 450 ± 50 °C, 40Ar/39Ar closure temperature of hornblende (Baldwin et al., 1990); Alaska (Fig. 12B). The 115–95 Ma feldspars overlap the less radiogenic part of and 300 ± 50 °C, 40Ar/39Ar closure temperature for biotite (Harrison et al., 1985). the field of feldspar data from the Late Devonian and Early Mississippian me- Late Triassic U-Pb SHRIMP ages indicate magmatic episodes ca. 215–212 taigneous rocks (Figs. 12A, 12B). The isotopic ratios for Devonian and Missis- and ca. 210–208 Ma (Table 5). Zircon and titanite U-Pb ages from different sippian rocks plot above the shale curve because these rocks contain signifi- parts of the Kechumstuk pluton are within analytical uncertainty, suggesting cant components of Archean and Proterozoic upper crustal material, as shown relatively rapid cooling from ≥900 °C at 215.5 ± 3.4 Ma through 660–700 °C by zircon core inheritance ages and detrital zircon ages of 3.3–1.1 Ga (Dusel-Ba- at 215.7 ± 3.1 Ma. A 207.9 ± 2.9 Ma SHRIMP age for leucotonalite west of the con and Williams, 2009). The one exception to Pb isotopic age grouping is the Kechumstuk pluton is within analytical uncertainty of 40Ar/39Ar ages of 207.8 ± anomalously radiogenic composition of feldspar from the 68 Ma sample (23) 1.2 and 205.6 ± 1.0 Ma for biotite and hornblende, respectively, from the same that plots within the area of 115–95 Ma mid-Cretaceous rocks (Fig. 12A). sample (A in Table 5) from the Kechumstuk pluton collected ~10 km south of Similar grouping of feldspar isotopic compositions by intrusive age is ev- the sample that yielded the 215.5 ± 3.4 Ma U-Pb age. It is not known whether ident in the 206Pb/204Pb versus 208Pb/204Pb plot (Fig. 12A); 208Pb/204Pb ratios are the 40Ar/39Ar ages indicate rapid cooling from 450 ± 50 °C to 300 ± 50 °C of positively correlated with increasing 206Pb/204Pb, indicating an increasing radio- the younger phase of the pluton at 207 Ma or slow cooling from ca. 215 Ma. genic component with decreasing age. Mid-Cretaceous (115–95 Ma) feldspars Geochronology from the Taylor Mountain batholith in the southeastern Eagle yielded the most radiogenic ratios, similar to feldspars from Late Devonian quadrangle suggests relatively rapid cooling from ≥900 °C to 660–700 °C at and Early Mississippian metaigneous rocks. 212 Ma for one sample, rapid cooling from ~450 °C to 300 °C at 211–209 Ma Pb isotopic compositions of 8 galena samples, 1 sphalerite sample, and 1 py- for another, and slow cooling from ~450 °C to 300 °C at 209–204 Ma for a third rite sample (including 5 new analyses and 5 reported in Dusel-Bacon et al., 2009) sample (samples C, D, and E in Table 5). from 4 Pb-Zn-Ag prospects in the western Fortymile district have 206Pb/204Pb ra- Intrusion of the pluton of Diamond Mountain in the earliest Jurassic (ca. tios of 19.301–19.521, 207Pb/204Pb ratios of 15.645–15.725, and 208Pb/204Pb ratios of 199 Ma) was followed by the intrusion of Early Jurassic granitoids in the west- 39.106–39.357 (Table 4). Pb isotopic compositions of sulfides in intrusion-related ern Fortymile district; SHRIMP U-Pb ages range from 190.5 ± 4.8 Ma for the mineralization, such as sulfides from proximal skarns, porphyry-type deposits, or small leucogranite body west of and at the base of the Mount Veta intrusion more distal veins, are commonly similar to those of igneous feldspars in genet- to 181.2 ± 2.6 Ma for K-feldspar megacrystic hornblende quartz monzonite ically related intrusions, assuming closed-system behavior (Tosdal et al., 1999). from the northeastern part of the intrusion (Fig. 4; Table 5). The other dated The Pb isotopic compositions of sulfide minerals from the LWM, Eva Creek, megacrystic Mount Veta sample, collected from the southern part of the intru- and Oscar prospects (Fig. 12C) plot below the shale curve and overlap the field sion, yielded an age of 186.2 ± 3 Ma; thus, given the analytical uncertainties of of feldspar Pb compositions of samples from the ca. 70 Ma Middle Fork cal- the two ages, it is not possible to determine the temporal relationship between dera, the ca. 66 Ma intrusion north of Mount Veta, and a feldspar from a ca. 72 the megacrystic phases of the intrusion and the other textural variants within Ma felsic porphyry at the Bluff Cu-Mo prospect in the eastern Tanacross quad- it. Hornblende from the same sample that yielded the 185.4 ± 1.2 Ma titanite rangle (Mortensen, unpub. data; Allan et al., 2013) (Fig. 12A). Pb isotopic com- U-Pb age gave an integrated 40Ar/39Ar age of 179.1 ± 1.1 Ma (B in Table 5), either positions of two galena samples from the Drumstick prospect have slightly requiring ~8.6–4.0 m.y. (depending on uncertainties) to cool from 660–700 °C higher 207Pb/204Pb values and plot near the feldspar compositions of a sample to ~450 °C, or reheating of the dated sample by the younger phases of the from the compositionally evolved ca. 110 Ma Corner granite and a sample from intrusion, such as that that yielded the ca. 181 Ma U-Pb age. Hornblende from the ca. 66 Ma granite near the West LWM prospect (Figs. 12A, 12C). quartz monzodiorite from the west-central part of the Mount Veta intrusion yielded a 188 ± 2 Ma 40Ar/39Ar integrated plateau age, within analytical uncertainty of the 190.5 ± 4.8 Ma zircon U-Pb age of a sample from the same area of DISCUSSION the intrusion and 186–188 ages from other parts of the Mount Veta intrusion, as well as the dike from the LWM drill core and the pluton at the Fish prospect Magmatism Viewed in Regional Context (Fig. 2; Table 5). These Early Jurassic U-Pb zircon ages of plutonic rocks from the south- Table 5 shows the U-Pb ages from our study (gray highlighting) together western Eagle quadrangle are within the range of ca. 197–185 Ma radiometric with previously determined radiometric ages of Mesozoic igneous rocks from ages of most of the Early Jurassic intrusions in the southeastern Eagle quad- various parts of the Eagle quadrangle, as well as a few relevant ages from rangle, as well as quartz monzonite that forms the ca. 183 Ma Seventymile the adjacent Big Delta and Tanacross quadrangles. The following closure tem- pluton in the northern Eagle quadrangle (Table 5). Biotite and hornblende from peratures are used in the discussion of the qualitative evaluation of cooling the same from the Seventymile pluton yielded closely constrained, overlap-

TABLE 5. NEW AND PREVIOUSLY DETERMINED U-Pb and 40Ar/39Ar AGES FOR IGNEOUS ROCKS FROM THE EAGLE QUADRANGLE, ALASKA Location Unit Rock type* Age† (Ma) Mineral, method Reference§ SW EA small body W of Mt. Veta intrusion Hbl Qtz diorite215.5 ± 3.4Zr, SHRIMP U-Pb 1 SW EA pluton of Kechumstuk Mountain Hbl Bt granodiorite 215.7 ± 3.1Ttn, TIMS U-Pb 2 SW EA pluton of Kechumstuk Mountain Hbl Bt granodiorite 215.0 ± 3.5Zr, SHRIMP U-Pb 1 SW EA pluton of Kechumstuk Mountain Hbl Bt granodiorite A 207.8 ± 1.2 Bt, 40Ar/39Ar 3 SW EA pluton of Kechumstuk Mountain Hbl Bt granodiorite A 205.6 ± 1.0Hbl, 40Ar/39Ar 3 SW EA drill core from LWM prospect Hbl Qtz diorite210 ± 3Zr, SHRIMP U-Pb 2 SW EA small body within Mt. Veta intrusion leucotonalite207.9 ± 2.9Zr, SHRIMP U-Pb 1 SW EA pluton of Diamond Mountain Hbl Bt granodiorite 201.0 ± 1.4Ttn, TIMS U-Pb 2 SW EA pluton n of Diamond Mountain Hbl Bt granodiorite 197.3 ± 0.7Hbl, 40Ar/39Ar 3 SW EA small body W of Mt. Veta intrusion leucogranite 190.5 ± 4.8Zr, SHRIMP U-Pb 1 SW EA Mt. Veta intrusion Hbl Qtz monzodiorite188 ± 2Hbl, 40Ar/39Ar 4 SW EA Mt. Veta intrusion Hbl Qtz monzonite 186.2 ± 3.0Zr, SHRIMP U-Pb 1 SW EA Mt. Veta intrusion Hbl Qtz monzonite B 185.4 ± 1.4Ttn, TIMS U-Pb 2 SW EA Mt. Veta intrusion Hbl Cpx monzonite 184.5 ± 3.0Zr, SHRIMP U-Pb 1 SW EA Mt. Veta intrusion Kfs megacrystic Hbl Qtz monzonite 181.2 ± 2.6Zr, SHRIMP U-Pb 1 SW EA Mt. Veta intrusion Hbl Qtz monzonite B 179.1 ± 1.1Hbl, 40Ar/39Ar 5 SW EA pluton at Fish prospect Hbl Qtz monzonite 187 ± 3Zr, SHRIMP U-Pb 2 SW EA felsic dike in LWM drill core Bt-bearing felsic dike 187.7 ± 2.3Zr, SHRIMP U-Pb 1 SW EA dike W of Mt. Veta intrusion Cpx granodiorite dike 184.8 ± 2.9Zr, SHRIMP U-Pb 1 SW EA sill east of Kechumstuk fault Hbl Bt granite porphyry 183.4 ± 3.6Zr, SHRIMP U-Pb 1 SW EA Corner granite leucogranite 111.8 ± 1.5Zr, SHRIMP U-Pb 1 SW EA Corner granite altered Bt granite 108.8 ± 1.7Zr, SHRIMP U-Pb 1 SE BD Mt. Harper batholith Bt granodiorite 110.5 ± 1.1Zr, SHRIMP U-Pb 6 SW EA Mt. Harper batholith Bt Hbl Qtz monzonite106.7 ± 0.6 Bt, 40Ar/39Ar 5 SW EA Mt. Harper batholith Bt granite105.8 ± 0.4 Bt, 40Ar/39Ar 3 SW EA Mt. Harper batholith Bt granite103.2 ± 1.5Zr, SHRIMP U-Pb 1 SW EA vein cutting Mt. Harper batholith secondary Ms in Qtz vein 102.7 ± 0.4 Ms, 40Ar/39Ar 3 (Section 21 prospect) SW EA dike cutting Mt. Harper batholith aplite 101.4 ± 1.4Zr, SHRIMP U-Pb 1 SW EA dike cutting Mt. Harper batholith granodiorite porphyry 96.2 ± 1.3Zr, SHRIMP U-Pb 1 SW EA Mt. Harper batholith (Lucky 13 granite 94.2 ± 0.3 Bt, 40Ar/39Ar 3 prospect) SW EA porphyry cutting Corner granite Qtz Fs Bt granitic porphyry 95.8 ± 1.5Zr, SHRIMP U-Pb 1 SW EA porphyry in Kechumstuk fault Qtz Pl rhyolite porphyry 93.9 ± 1.3Zr, SHRIMP U-Pb 1 SW EA drill core from Fish prospect rhyolite porphyry 70.5 ± 1.1Zr, SHRIMP U-Pb 2 SW EA Middle Fork Caldera outﬂow tuff71.1 ± 0.5Zr, SHRIMP U-Pb 7 SW EA Middle Fork Caldera intracaldera tuff 70.0 ± 1.2Zr, SHRIMP U-Pb 7 SW EA Middle Fork Caldera granite porphyry 69.7 ± 1.2Zr, SHRIMP U-Pb 7 SW EA Middle Fork Caldera outﬂow tuff 69.1 ± 0.5 Bt, 40Ar/39Ar 8 SW EA dike within Kechumstuk fault Qtz Kfs rhyolite porphyry 68.1 ± 0.8Zr, SHRIMP U-Pb 1 SW EA intrusion north of Mt Veta Bt Hbl granite 67.9 ± 1.1Zr, SHRIMP U-Pb 1 SW EA intrusion within Kechumstuk fault Fine-grained Bt granite 67.7 ± 0.7Zr, SHRIMP U-Pb 1 SW EA intrusion north of Mt Veta Bt Hbl granite 65.8 ± 1.5Zr, SHRIMP U-Pb 1 SW EA intrusion north of Mt Veta Bt granite65.8 ± 1.4Zr, SHRIMP U-Pb 1 SW EA dike within Kechumstuk fault aplite ≤ ~65Zr, SHRIMP U-Pb 1 C TA Mt. Fairplay Ksp Cpx Hbl Bt syenite 67.0 ± 1.5Zr, SHRIMP U-Pb 1 C TA Mt. Fairplay Hbl Bt Qtz monzonite66.5 ± 1.1Zr, SHRIMP U-Pb 1 C TA Mt. Fairplay Hbl Bt Qtz monzonite67.2 ± 2 Bt, K-Ar 9 (continued )

TABLE 5. NEW AND PREVIOUSLY DETERMINED U-Pb and 40Ar/39Ar AGES FOR IGNEOUS ROCKS FROM THE EAGLE QUADRANGLE, ALASKA (continued ) Location Unit Rock type* Age† (Ma) Mineral, method Reference§ SE EA Taylor Mountain batholith Hbl Bt Qtz monzodiorite C 212 ± 3.3Zr, SHRIMP U-Pb 1 SE EA Taylor Mountain batholith Hbl Bt Qtz monzodiorite C 212Ttn, TIMS U-Pb 10 SE EA Taylor Mountain batholith Hbl Bt Qtz monzodiorite D 210.5 ± 1.6 Bt, 40Ar/39Ar 11,12 SE EA Taylor Mountain batholith Hbl Bt Qtz monzodiorite D 209.5 ± 1.5Hbl, 40Ar/39Ar 11,12 SE EA Taylor Mountain batholith Hbl Bt Qtz monzodiorite E 209Hbl, 40Ar/39Ar 4 SE EA Taylor Mountain batholith Hbl Bt Qtz monzodiorite E 204 Bt, 40Ar/39Ar 4 SE EA small intrusion, Taylor Hwy Ep-bearing Ms Bt granite 197.2 ± 2.5Zr, TIMS U-Pb 2 SE EA small intrusion, Fortymile River Ep-bearing Bt Ms granite ~197 Zr, TIMS U-Pb 2 SE EA small intrusion, Fortymile River Ep-bearing leucogranite 196 ± 4Zr, SHRIMP U-Pb 13 SE EA Pig pluton Hbl Qtz monzonite F 193.2 ± 0.4Zr, TIMS U-Pb 2 SE EA Pig pluton Hbl Qtz monzonite F 172.5 ± 0.6Ttn, TIMS U-Pb 2 SE EA Pig pluton Hbl monzodiorite 194 ± 2Hbl, 40Ar/39Ar 14 SE EA Pig pluton Hbl Cpx monzodiorite 188.1 ± 1.4 Bt, 40Ar/39Ar 5, SE EA Mt. Warbelow pluton Ms Bt granite 191.3 ± 0.4Zr, TIMS U-Pb 5,2 SE EA Mt. Warbelow pluton Bt granite185 ± 1 Bt, 40Ar/39Ar 14 SE EA Uhler pluton Ep-bearing Bt Hbl granodiorite 188.0 ± 1.0Hbl, 40Ar/39Ar 11 SE EA Napoleon Creek pluton Hbl monzonite G 188.4 ± 1.2Zr, TIMS U-Pb 2 SE EA Napoleon Creek pluton Hbl monzonite G 187.3 ± 0.6Ttn, TIMS U-Pb 2 SE EA Napoleon Creek pluton Hbl Bt Qtz monzonite186.5 ± 1.0Hbl, 40Ar/39Ar 11 SE EA Chicken pluton Bt Hbl Qtz diorite 187.8 ± 0.9Hbl, 40Ar/39Ar 11 SE EA small intrusion, Taylor Hwy Bt Hbl granodiorite 186.7 ± 1.6Ttn, TIMS U-Pb 2 SE EA Walker Fork pluton Bt Hbl granodiorite 105.2 ± 2.0Ttn, TIMS U-Pb 2 SE EA Walker Fork pluton Hbl Bt granodiorite 99.0 ± 0.5 Bt, 40Ar/39Ar 11 C EA Happy granite vein Ms 214.4 ± 0.6 Ms, 40Ar/39Ar 3 C EA Butte Creek hornblendite Cpx Bt hornblendite 184.1 ± 0.6Hbl, 40Ar/39Ar 3 C EA Ruby Creek granite granite 102.1 ± 0.4 Bt, 40Ar/39Ar 3 N EA Seventymile pluton Qtz monzodiorite H 183.3 ± 0.6Hbl, 40Ar/39Ar 3 N EA Seventymile pluton Qtz monzodiorite H 183.6 ± 0.6 Bt, 40Ar/39Ar 3 N EA Upper Granite Creek granite 93.3 ± 0.5 Bt, 40Ar/39Ar 3 Note: Abbreviations: EA—Eagle quadrangle; BD—Big Delta quadrangle; TA—Tanacross quadrangle; SW—southwestern; SE—southeastern; N—northern; C— central; Mt.—Mount; W—west; LWM—Little Whiteman; Bt—biotite; Cpx—clinopyroxene; Ep—epidote; Hbl—hornblende; Ksp—K-feldspar; Ms—muscovite; Qtz— quartz; Ttn—titanite; Zr—zircon. Acronyms: TIMS—thermal ionization mass spectrometer; SHRIMP—sensitive high-resolution ion microprobe. *Uppercase bold letters after rock names indicate that the ages listed for those were determined for mineral separates from the same sample. †Analytical uncertainties are 2σ for U-Pb ages and 1σ for 40Ar/ 39Ar and K-Ar ages. §References for radiometric dates: 1— this study; 2—Dusel-Bacon et al. (2009); 3—Newberry et al. (1998a); 4—Cushing (1984); 5—Dusel-Bacon et al. (2002); 6—Day et al. (2007); 7—Bacon et al. (2014); 8—Bacon and Lanphere (1996); 9—Wilson et al. (1985); 10—Aleinikoff et al. (1981); 11—Werdon et al. (2001); 12—M.B. Werdon (2014, written commun.); 13—Day et al. (2002); 14—Szumigala et al. (2000).

ping ages (I in Table 5), indicating rapid cooling from ~450 to 350 °C at 183 Ma. rocks (Fig. 12), indicating a smaller ratio of mantle to crustal source material Zircon, titanite, and hornblende ages from the Napoleon Creek pluton indicate than was present during generation of Late Triassic magmas. rapid cooling from >900 °C to 660–700 °C (H in Table 5) at 188 Ma, with another Two phases of the mid-Cretaceous magmatism that postdated the Early sample cooling through ~450 °C at 187 Ma (Table 5). Crystallization ages by Jurassic and Early Cretaceous dynamothermal metamorphic episodes that af- different methods and from different locations in the Pig and Mount Warbe- fected the Yukon-Tanana Upland (Hansen and Dusel-Bacon, 1998; Dusel-Bacon low plutons vary considerably (Table 5) and likely indicate either unrecognized et al., 2002) are present southwest of the Mount Veta ridge and in the Mount composite phases to the plutons or complexities in mineral cooling histories. Harper area to the west (Fig. 3; Table 5). The older phase (ca. 112–101 Ma) is Feldspars from Early Jurassic rocks have Pb isotopic ratios intermediate be- represented by the Corner granite and Mount Harper batholith and dikes in- tween those of the Late Triassic and earliest Jurassic rocks and the Cretaceous trusive into the batholith. U-Pb zircon ages of 111.8 ± 1.5 and 108.8 ± 1.7 Ma

from the Corner granite are identical within analytical uncertainty. U-Pb zircon (Breitsprecher and Mortensen, 2004; Gordey and Ryan, 2005; Allan et al., 2013), and biotite 40Ar/39Ar ages from different localities within the Mount Harper ba- were roughly contemporaneous with Late Cretaceous magmatism in the For- tholith range from 110.5 ± 1.1 to 103.2 ± 1.5 Ma; late-stage mineralized quartz tymile district. veins and aplite dikes formed at 102.7 ± 0.4 and in 101.4 ± 1.4 Ma, respectively. The ca. 65 Ma or younger microporphyritic aplite dike from in the southern The Mount Harper batholith is probably composite in nature (it has only been Kechumstuk fault zone has an arc geochemical signature comparable to that mapped in a reconnaissance manner) and it is therefore not possible to know of the Late Cretaceous felsic rocks in the western Fortymile area. We therefore whether different ages reflect different pulses of magmatism or differences in consider it more likely related to the end stage of that magmatic episode than mineral cooling ages or other aspects of the geochronology. Granitoids of this to subsequent Paleocene and Eocene (ca. 60–50 Ma within-plate magmatism age make up the Walker Fork pluton in the southeastern Eagle quadrangle and that produced small volumes of comagmatic felsic plutons and mafic dikes, Ruby Creek granite in the central Eagle quadrangle (Table 5) and are present in bimodal mafic-felsic volcanic rocks, and shallow intrusions that are present the southeastern Big Delta quadrangle to the west and adjacent Yukon (Day et sporadically in much of interior Alaska and Yukon (Bacon et al., 1990; Foster et al., 2003; Dilworth et al., 2007; Werdon et al., 2004b; Hart et al., 2004, and refer- al., 1994; Newberry, 2000). ences therein) (Fig. 2). The extrusive equivalent of this earlier mid-Cretaceous Elsewhere in the northern Cordillera, Late Cretaceous igneous rocks display a magmatic episode is preserved in rhyolite calderas in the Tanacross quadran- variety of non-arc geochemical compositions and tectonic signatures. Newberry gle (Bacon et al., 1990) (Fig. 2) that yield preliminary laser ablation U-Pb zircon (2000) showed that Late Cretaceous plutonic rocks from interior Alaska outside ages of ca. 110 and 108 Ma for welded tuff (Mortensen and Dusel-Bacon, 2014). of the western Fortymile area include both a 78–60 Ma peraluminous group with The ca. 96–94 Ma phase of early-Late Cretaceous magmatism is repre- syncollisional tectonic signatures and a 71–68 Ma metaluminous group with sented in the southwestern Eagle quadrangle by a small intrusion and quartz within-plate tectonic signatures. In southwestern Yukon, volcanic rocks of the feldspar porphyry dikes that crosscut the intrusions or country rock in or near Carmacks Group exhibit an even broader compositional variation that includes the southern part of the Kechumstuk fault zone and dikes in the Mount Harper K- and Mg-rich (shoshonitic) basalt and basaltic andesite (Johnston et al., 1996) batholith (Figs. 3 and 4) and the ca. 93 Ma Upper Granite Creek intrusion in and lesser calc-alkalic intermediate and felsic volcanic rocks (Tempelman-Kluit, the northern Eagle quadrangle (Table 5). A northeast-trending quartz feldspar 1974; Ryan and Gordey, 2002; Colpron and Ryan, 2010, and references therein). dike west of the study area in the southeastern Big Delta quadrangle yielded a comparable SHRIMP U-Pb zircon age of 95.4 ± 0.9 Ma (Day et al., 2007). Plutons in this age range are present in the Yukon-Tanana Upland west of Age of Epigenetic Mineralization in the Western Fortymile District the study area, as are dikes and small mafic bodies dated as ca. 93 Ma (e.g., Dilworth et al., 2007; Werdon et al., 2004b; Hart et al., 2004). As with the older Base and precious metal epigenetic prospects in the Mount Veta area are Cretaceous age group, whole-rock geochemistry of the ca. 96–94 Ma rocks in- spatially associated with both Early Jurassic and mid- and Late Cretaceous dicates granitic, high-K calc-alkalic, and peraluminous compositions and an intrusions and dikes (Figs. 3 and 4). Although we did not determine the time of arc origin, but their 207Pb/204Pb and 208Pb/204Pb isotopic ratios are slightly lower mineralization directly, a comparison of Pb isotopic data for igneous feldspars than those in the 115–95 Ma rocks, implying a smaller crustal component in from dated intrusions and prospect sulfides indicates that mineralization prob- the younger Cretaceous magmas. ably was associated with Late Cretaceous magmatism that produced the ca. 70 Late Cretaceous felsic magmatism in the Fortymile district produced the ca. Ma Middle Fork caldera or the 68–66 Ma granitic intrusions and dikes. Galena 70 Ma intracaldera tuff, granite porphyry, and outflow tuff of the Middle Fork Pb isotopic compositions from the Drumstick prospect also overlap feldspar Pb caldera (Bacon et al., 2014) and altered rhyolite porphyry intersected in drill ratios from one of the samples from the ca. 110 Ma Corner granite, allowing the core from the Fish prospect (Dusel-Bacon et al., 2009) (Table 5). These ages are possibility of a previous mid-Cretaceous mineralizing episode at that locality. within analytical uncertainty of our SHRIMP U-Pb zircon crystallization ages Although Pb isotopic ratios from sulfides from Fortymile prospects also over- for the intrusion north of Mount Veta, the intrusion and dike associated with lap those of feldspars from 94 to 78 Ma intrusive rocks from east-central Alaska the Kechumstuk fault, and the younger phase of the intrusion north of Mount outside the study area, the absence of feldspar Pb isotope data from newly Veta (Table 5). Zircon ages of 67.0 ± 1.5 and 66.5 ± 1.1 Ma from the Mount Fair- dated ca. 96–94 Ma felsic porphyries associated with the Kechumstuk fault play intrusion in the southern Fortymile district in the Tanacross quadrangle (24 (Fig. 4) precludes our evaluation of this as a possible phase of intrusion-related and 25 in Fig. 2) are within analytical uncertainty of the ca. 70–66 Ma ages for sulfide mineralization. The paucity of lithologically similar porphyries noted felsic magmatism in the Mount Veta area. Quartz monzonite from the Mount during mapping of the Kechumstuk fault area suggest that possible products Fairplay intrusion yielded a biotite K-Ar age of 67 ± 2 Ma (Wilson et al., 1985), of this magmatic episode are volumetrically small and thus unlikely to be the indicating rapid cooling from >900 °C to ~350 °C at ca. 67 Ma (Table 5). Eruption causative intrusions for the epigenetic mineralization. of volcanic rocks of the Carmacks Group and intrusion of the small, high-level, Intense alteration in the 108.8 ± 1.7 Ma Corner granite sample and ca. 96–94 72–67 Ma Prospector Mountain plutonic suite in southwestern Yukon, Canada Ma porphyries and deformed quartz in the Corner sample (Supplemental File

1 [see footnote 1]) suggest the presence of hydrothermal fluids and movement 66 Ma intrusions with some of the prospects noted here. We suggest that this along splays of the Kechumstuk sinistral-normal fault zone and in faults west gas loss likely records Late Cretaceous mineralization by fluids at a tempera- of the Mount Veta ridge (Fig. 4), either synchronous with intrusion of one or ture below the closure temperature of the Ar system and sericitic muscovite. both of the mid-Cretaceous phases of felsic magmatism or during subsequent The Late Cretaceous timing of carbonate replacement and skarn mineral- intrusion of the 68–65 Ma rhyolite dikes that parallel the faults. We propose that ization in the Mount Veta area overlaps with the time of porphyry-related min- the best indication for the timing of hydrothermal fluid flow and associated eralization to the south and east. Newberry et al. (1998a) proposed that a north- mineralization for the western Fortymile base and precious metal prospects west-trending belt of ca. 70 Ma Cu-Mo ± Au porphyry prospects and deposits is the proximity of the prospects to 68–66 Ma intrusions and dikes (Fig. 4). The of the Carmacks belt occurs in west-central Yukon and extends northwestward Oscar skarn prospect is located within ~300 m of both the Early Jurassic Mount into the Tanacross quadrangle. At the Mosquito porphyry Cu-Mo-Au prospect Veta intrusion and the 65.8 ± 1.5 Ma intrusion north of Mount Veta (Fig. 4). Fur- at the northwestern end of this belt, south of our study area (prospect C, Fig. ther south, narrow, undated felsic dikes mapped adjacent to northeast-trend- 2), vein K-feldspar yielded a 40Ar/39Ar age of 70.0 ± 0.3 Ma interpreted as the ing faults 3 km north of and along strike from the Eva prospect (Day et al., 2014) age of mineralization (Newberry et al., 1998a). Newberry et al. (1998a) pointed appear compositionally similar to the ca. 68 Ma dike along the Kechumstuk out that this age was comparable to that in the Mount Fairplay alkalic complex fault to the east (Fig. 4), suggesting a likely Late Cretaceous age for the intru- that contains minor gold-bearing veins, and proposed that mineralization at sion-related mineralization. A 3-km-long 68.1 ± 0.8 Ma Late Cretaceous felsic Mount Fairplay was part of the Carmacks belt. Our zircon U-Pb ages of ca. 67 dike occurs within a splay of the Kechumstuk fault zone ~100 m west of the Ma confirm a Late Cretaceous, albeit slightly younger, age for Mount Fairplay Mitchell prospect (Fig. 4) and leucocratic dikes are present at the prospect (Full magmatism. Northeast of Mount Fairplay, granodiorite that crops out in the Metal Minerals, USA, Inc., 2009, in-house report). same general vicinity as the Pika Canyon porphyry Cu prospect (prospect D, Dip-slip movement along the faults at the LWM played an important role in Fig. 2) yielded a zircon U-Pb age of ca. 70 Ma (Mortensen, 2000, unpub. data), juxtaposing unreactive metavolcanic footwall rocks against reactive carbonate suggesting that this prospect is also part of the Late Cretaceous Carmacks hanging-wall rocks and channeling metalliferous hydrothermal fluids to their porphyry belt. Host rocks to Cu-Mo porphyry-style mineralization at the Bluff sites of carbonate replacement (Siron et al., 2010). The left-lateral jog in the Ke- and adjacent Taurus prospects in the eastern Tanacross quadrangle (prospect chumstuk fault that formed the pull-apart releasing bend near the LWM pros- E, Fig. 2) and the giant Casino Cu-Mo-Au porphyry deposit in Yukon (south of pect also facilitated mineralization (Siron et al., 2010). The location of the West area shown in Fig. 2) yielded slightly older zircon U-Pb ages of 74–72 Ma (Allan LWM prospect within strands of the Kechumstuk fault system suggests a sim- et al., 2013). Both in easternmost Alaska and southwestern Yukon, these pros- ilar role for fluid flow along the faults; we consider it likely that mineralization pects are proposed to be genetically associated with northeast-trending faults there was coeval with that at the LWM. At the Eva Creek prospect, calcite-filled (Fig. 2) (Sanchez et al., 2013; Allan et al., 2013), consistent with the results of our breccia zones as much as 17.7 m thick in drill core and northeast-trending study indicating a Late Cretaceous age for epithermal mineralization and its metal anomalies in soils (Dashevsky et al., 1986) are parallel to local faults, association with sinistral and normal movement along the Kechumstuk fault. suggesting a spatial, and likely genetic, association between deformation and Mid-Cretaceous SHRIMP ages for igneous rocks associated with skarn and mineralization. A similar association of Late Cretaceous epithermal prospects porphyry Cu-Mo-W-Ag prospects within the Mount Harper batholith in the with northeast-trending faults has been shown in metallogenic and geophysi- western part of our study area (Fig. 2) indicate that mineralization accompa- cal studies spanning southwestern Yukon and easternmost Alaska (Allan et al., nied ca. 103 Ma magmatism, in accordance with previous studies in the area 2013; Sanchez et al., 2013). and elsewhere in east-central Alaska (Newberry et al., 1998a; Newberry, 2000). At the LWM carbonate replacement prospect, Siron et al. (2010) noted that At the Section 21 prospect, the correspondence between our 101.4 ± 1.4 Ma altered feldspar porphyry dikes are closely associated with, and often in con- zircon U-Pb age for the aplite dike (Fig. 3) and the 102.7 ± 0.4 Ma 40Ar/39Ar age tact with, massive sulfides, and postulated that the dikes may have acted as of muscovite from a mineralized vein (Newberry et al., 1998a) indicates that aquitards, focusing metalliferous hydrothermal fluids along nearly vertical intrusion of felsic dikes and mineralized quartz veins was nearly synchronous. structures. Our U-Pb zircon date of 187.7 ± 2.3 Ma for one of these dikes (6, Ta- A comparable age for mineralization at the Peternie Cu-Mo porphyry prospect, ble 2; Fig. 5), together with the similar, albeit less robust (28% of 39Ar released), ~70 km to the southeast (prospect A, Fig. 2; Werdon et al., 2004a) is suggested 40Ar/39Ar incremental heating age of 187.5 ± 2.0 Ma for secondary sericitic mus- by a 102.8 ± 0.5 Ma 40Ar/39Ar age for secondary K-feldspar in a vein that cuts covite from an altered porphyry dike in LWM drill core (Supplemental File 4 an intrusion that is spatially associated with the Sixtymile Butte caldera; a ca. [see footnote 4]), suggests that Early Jurassic dikes may have played a role in 110–108 Ma zircon U-Pb age for welded tuffs from the Sixtymile Butte and ad- establishing structural pathways that helped focus the Late Cretaceous hydro- jacent calderas (Mortensen and Dusel-Bacon, 2014) provides a maximum age thermal fluids. The minor Ar gas loss at ca. 65 Ma in the sample of secondary for mineralization at the Peternie prospect. sericitic muscovite is consistent with the Late Cretaceous age of mineralization Mid-Cretaceous hydrothermal activity in the western Fortymile district also permitted by Pb isotopic data and inferred from the spatial association of 68– overlaps a 104 Ma Re-Os molybdenite age (Selby et al., 2002) that best deter-

mines the timing for orogenic and/or intrusion-related Au mineralization within tral shear in the northeast-trending Black Mountain tectonic zone in the south- 109–103 Ma, reduced, ilmenite-series igneous rocks at the Pogo mine (Fig. 2) western Big Delta quadrangle (Fig. 2) began in mid-Cretaceous time, based west of our study area (Rhys et al., 2003; Dilworth et al., 2007). In southern on the spatial association of ca. 110–95 Ma granitoids and related felsic dikes, Yukon, south of the area shown in Figure 2, metallogenic episodes, including plugs, and quartz vein systems with the tectonic zone, with recurrent move- Au-bearing breccia complexes, skarns, and polymetallic veins, are associated ment taking place in early Tertiary and Quaternary time. with 115–98 Ma magnetite-series arc magmas in the Dawson Range (Allan et We propose that mid- to Late Cretaceous northwest-trending dextral faults al., 2013, and references therein). existed outboard of the domain of northeast-trending oblique-sinistral faults in A younger, ca. 96–92 Ma, Cretaceous phase of epigenetic mineralization is east-central Alaska and that, like the subsequently developed Denali and Tin- documented in the intrusion-related prospects of the Mount Harper area and tina fault systems, caused rotation and wrenching of the inboard crust along elsewhere in east-central Alaska. The 96.2 ± 1.3 Ma SHRIMP U-Pb age of the the Kechumstuk and other northeast-trending normal-sinistral faults. Mid-Cre- porphyry dike at the Section 21 prospect (Fig. 3) corresponds with the timing of taceous dextral, orogen-parallel displacement has been recognized along mineralization at the nearby Lucky 13 W-rich skarn on Larsen Ridge (prospect northwest-trending faults in the northern Cordillera in Canada (Gabrielse et al., A, Fig. 3) proposed by Newberry et al. (1998a) on the basis of a 94.2 ± 0.3 Ma 2006; Nelson et al., 2013). The age of these faults in Canada is constrained by biotite 40Ar/39Ar age of an associated granite. At the Fort Knox gold deposit near ca. 115–100 Ma ages of synkinematic and late kinematic granitic plutons bor- Fairbanks (Fig. 2), mineralization consists of gold- and sulfide-bearing quartz dering the faults and ca. 110–95 Ma K-Ar dates on muscovite generated during veins and associated alteration within a composite stock dated as 92.5 ± 0.2 fault movement (Gabrielse et al., 2006). Ma and 92.4 ± 1.2 Ma by zircon U-Pb and molybdenite Re-Os, respectively Normal faults along the margins of the 70 Ma Middle Fork caldera, result- (Selby et al., 2002, and references therein), confirming an intrusion-related ori- ing in preservation of caldera fill (Fig. 3), may have developed during caldera gin for gold-bearing fluids (McCoy et al., 1997). Skarn and associated Pb-Zn-Ag formation or followed it by several million years. A broad temporal association carbonate replacement mineralization of the Lead Creek and Champion Creek between caldera formation, normal faults, and a Late Cretaceous phase of the skarn prospects in the eastern Fortymile mining district (prospect B, Fig. 2) northwest-side-up normal dip-slip and sinistral strike-slip (oblique transten- also are attributed to this ca. 96–92 Ma magmatic episode on the basis of a sional) displacement along the Kechumstuk fault zone is suggested by U-Pb TIMS zircon U-Pb age of 96.2 ± 1.0 Ma for felsic porphyry associated with min- zircon ages of 68.1 ± 0.8 and 67.7 ± 0.7 Ma for the felsic dike and granitic stock eralized intervals in Lead Creek drill core and galena Pb isotopes from the two within strands of the Kechumstuk fault. The fault zone constitutes the western prospects that are within the range of feldspars from mid- and Late Cretaceous margin of the upthrown subvolcanic intrusion, relative to the downdropped intrusions (Dusel-Bacon et al., 2003). Middle Fork caldera. Analogous northeast-trending faults and grabens of inferred Late Cretaceous age are spatially associated with Carmacks Group volcanics in western Yukon (Mortensen, 1996; Allan et al., 2013). Tectonic Framework of Magmatism Several lines of evidence indicate recurrent movement along the Kechum- stuk fault zone. Graphitic quartzite fault breccia from drill holes along the Ke- Northeast-Trending Faulting chumstuk fault zone at the LWM prospect consists of subrounded to angu- lar fragments of unaltered marble, dolomitized marble, altered and strongly Wilson et al. (1985) proposed the existence of northeast-trending linea- pyritized dacite porphyry dike (likely coeval with our 187.7 ± 2.3 Ma sample), ments that divided the bedrock of the Yukon-Tanana Upland into different do- and rare Zn- and Pb-sulfide mineral clasts in a matrix of graphite with minor mains, based on the regional distribution of K-Ar ages. Subsequent geologic disseminated pyrite. This texture indicates that the Kechumstuk faults under- and aeromagnetic data have confirmed and expanded on the significance of went movement after dacite porphyry dike emplacement and sulfide miner- the lineaments and identified them as steeply dipping faults with sinistral and alization (Siron et al., 2010). Recurrent fault movement is also evident in the oblique-extensional dip-slip displacement (e.g., Dusel-Bacon and Murphy, narrow zones of brittle deformation in northeast-trending faults on either side 2001; O’Neill et al., 2010; Sanchez et al., 2013; Allan et al., 2013). Movement of the Mount Veta ridge (Fig. 4; Day et al., 2014) that contain evidence of su- along the northeast-trending faults in the Yukon-Tanana Upland (Fig. 2) has perimposed offset of preexisting brittle deformation, rebrecciated silicic and been attributed to the clockwise rotation of blocks resulting from dextral shear iron-oxide alteration, and brecciation of younger granitic and rhyolitic dikes along the Tintina and Denali fault systems (e.g., Page et al., 1995; O’Neill et al., and small intrusions. 2010). Most movement along these faults is considered to have taken place in We speculate that the parallelism and spatial proximity of the Kechumstuk early Tertiary time (e.g., Gabrielse et al., 2006), but in Alaska both faults sys- fault zone to the steep, southeast-dipping Early Jurassic Mount Veta intrusion tems show evidence for displacement continuing into the Holocene (Stout et and associated dikes revealed in LWM drill core may indicate that Early Jurassic al., 1973; Plafker and Berg, 1994) and, in the Denali fault, to the present (Eber- structures associated with synkinematic intrusion and inferred northwest-ver- hart-Phillips et al., 2003). O’Neill et al. (2010) postulated that normal and sinis- gent contractional deformation based on kinematic studies elsewhere in the

YTT (Hansen and Dusel-Bacon, 1998) may have been reactivated by Cretaceous (Fig. 13B). Feldspars from Late Triassic rocks have the lowest Pb isotopic ratios and younger sinistral and normal faulting. The Black Mountain tectonic zone in of any of the analyzed Mesozoic igneous rocks (Fig. 12) indicating the largest ra- the Big Delta quadrangle is another example of structural reactivation following tio of mantle to crustal components in their source areas. The mantle and upper mid-Cretaceous faulting in which the tectonic zone served as a tectonic conduit crustal components of Pb isotopes for feldspars from Late Triassic intrusions in for younger mafic dikes and a Paleocene rhyolite flow-dome complex, with -de the Fortymile district likely originated from Pb derived from subduction-related formation continuing into the Quaternary (Day et al., 2007; O’Neill et al., 2010). mantle and from upper crustal material that formed the basement of both the An even older history for the zone of tectonic weakness associated with parautochthonous YTa and the rifted allochthonous YTT. the northeast-trending Kechumstuk fault zone and associated faults west of Early Jurassic magmatism in east-central Alaska accompanied a subse- the Mount Veta ridge is suggested by the occurrence of the metaharzburgite quent episode of east-dipping subduction under the reconfigured continental lens (described above) located between northeast-trending faults and near the margin (Fig. 13C) (Beranek and Mortensen, 2011). In the southeastern Eagle western boundary of the YTT and parautochthonous YTa (Fig. 3). In Dusel-Ba- quadrangle, weakly foliated Early Jurassic plutons yield U-Pb zircon and ti- con et al. (2013), it was proposed that the metaharzburgite lens in the Mount tanite ages between ca. 197 and 186 Ma (Table 5). Magmatic epidote is present Veta area, like those in other parts of the Yukon-Tanana Upland, are pieces of in three ca. 197 Ma intrusions and one 188 Ma intrusion (Table 5), indicating the mantle that originated from beneath the Seventymile ocean basin and/ emplacement of the host plutons at mesozonal crustal depths of >15 km (Wer- or from subcontinental mantle lithosphere of the allochthonous YTT or the don et al., 2001; Day et al., 2002; Dusel-Bacon et al., 2009). Magmatic epidote western margin of Laurentia and were tectonically emplaced within crustal also occurs in the synkinematic and synmetamorphic Early Jurassic (186.0 ± rocks during closure of the Seventymile ocean basin in late Paleozoic to early 2.8 Ma) foliated Aishihik batholith that intrudes rocks equivalent to the YTT in Mesozoic time. They speculated that, given the complex Paleozoic and Meso- southwest Yukon (Johnston and Erdmer, 1995). zoic structural and thermal evolution of the allochthonous YTT, outlined here, Kinematic, metamorphic, and geochronologic studies from the southeast- exposure of the metaharzburgite lens between strands of northeast-trending ern Eagle quadrangle (Hansen and Dusel-Bacon, 1998; Dusel-Bacon et al., 1995, faults with predominantly Late Cretaceous to early Tertiary sinistral and/or nor- 2002, 2009) indicate that intrusion of the foliated Early Jurassic granitoids was mal movement suggests that the northeast-trending faults may have reused synkinematic to late kinematic with northwest-vergent (orogen-parallel) con- the Paleozoic or early Mesozoic deep crustal structure formed during internal traction that emplaced the Fortymile River assemblage onto the Nasina assem- deformation and tectonic transport of the oceanic and pericratonic terranes. blage to the north and the Lake George assemblage to the south. The above studies attribute this contraction to crustal thickening and a major period of moderate- to high-pressure (7–12 kbar) amphibolite facies metamorphism of Permian–Cretaceous Tectonic Evolution rocks in both the parautochthonous Yukon Tanana assemblage and the allochthonous YTT (Fig. 13C). In situ SHRIMP monazite geochronology and pres- Progressive closure of the Slide Mountain–Seventymile ocean occurred sure-temperature data from rocks of the allochthonous YTT in western Yukon along its western margin in middle to late Permian time as a result of a change indicate an equivalent Early Jurassic (ca. 195–187 Ma) metamorphic episode in plate motion that led to southwest-dipping right-oblique subduction be- that is interpreted to reflect the change from regional contact metamorphism neath the allochthonous YTT (e.g., Dusel-Bacon et al., 2006; Nelson et al., during arc plutonism to internal duplication of the YTT during its collision with 2006; Beranek and Mortensen, 2011) (Fig. 13A). U-Pb geochronologic studies the North American craton (Berman et al., 2007). We propose that develop- in parts of the YTT in Yukon identified a major fabric-forming metamorphic ep- ment of the southeast-dipping thrust fault east of the Kechumstuk fault zone isode that accompanied late Permian crustal thickening (Berman et al., 2007; and intrusion of the tabular, synkinematic Early Jurassic Mount Veta intrusion Beranek and Mortensen, 2011). The 215.0 ± 3.5 Ma SHRIMP U-Pb age for the and the southeast-dipping dikes identified in LWM drill core accompanied the pluton of Kechumstuk Mountain that is discordant to the regional penetrative northwest-vergent contraction. The 183.4 ± 3.6 Ma age for the intensely altered, fabric establishes a minimum age for the oldest metamorphism recorded in but nonfoliated, hornblende-biotite granite porphyry that crops out as a thin the allochthonous YTT in east-central Alaska. sill in the hanging wall of the thrust fault that places Nasina assemblage rocks Following closure of the Slide Mountain–Seventymile ocean in the late over Fortymile River assemblage rocks may provide an upper age limit for the Permian, subduction stepped outboard, leading to Triassic magmatism above a thrusting and attendant shortening in the western Fortymile area. The Early newly established east-dipping subduction zone beneath the now reconfigured Jurassic dikes at the LWM prospect and possibly the contemporaneous syn- western margin of North America from Alaska and Yukon to California (Dickin- tectonic Mount Veta intrusion may have played a role in subsequent epigenetic son, 2004; Nelson and Colpron, 2007; Beranek and Mortensen, 2011; Nelson mineralization by providing structural pathways that helped focus the Late Cre- et al., 2013). In east-central Alaska and the northern Canadian Cordillera, Late taceous metalliferous hydrothermal fluids along the steep northeast-trending Triassic arc plutons are restricted to the allochthonous YTT (Fig. 2), suggesting faults that likely reused the zones of weakness established during Early Juras- that the parautochthonous YTa was east or north of the zone of arc magmatism sic contraction.

A. Mid-Permian EXPLANATION EAST WEST Pv Oceanic lithosphere of the Seventymile–Slide Mountain terrane Oceanic lithosphere of other terranes

Nasina assemblage YT T B. Late Triassic Fortymile River assemblage and Chicken metamorphic complex WEST Pv EAST Trg Metasedimentary and metavolcanic rocks

pY Ta Augen gneiss of Lake George assemblage

North American crust

C. Early Jurassic TTNW contraction Jg SOUTHWEST NORTHEAST Pv Trg Figure 13. Schematic diagram for Permian to Late Cretaceous thermal and deformational evolution of east-central Alaska. (A) Mid-Permian. West-dipping subduction beneath the outboard, al- lochtonous Yukon-Tanana terrane (YTT). (B) Late Triassic. Closure of most, if not all, of the Seventymile–Slide Mountain ocean, development of a new east-dipping subduction zone, and associated Late Triassic plutonism in the YTT. (C) Early Jurassic. Continued subduction beneath the continental margin results in orogen-parallel (top-to-the-northwest; TTNW) thrusting, internal imbrication within both upper plate YTT and lower plate parautochthonous Yukon-Ta- nana assemblage (pYTa), and intrusion of synkinematic to slightly D. Mid-Cretaceous TTSE extension postkinematic intrusions (modified from Dusel-Bacon et al., 2002). Rapid cooling of upper plate rocks ca. 186 Ma. (D) Mid-Cretaceous. SOUTHWEST NORTHEAST Northeast-dipping subduction results in collision of the Wrangellia composite terrane (WR) terrane, extension, and exhumation of low- WR mKg mKg er plate pYTa during orogen-parallel top-to-the-southeast (TTSE) deformation. Postkinematic intrusion of mid-Cretaceous granitoids and formation of coeval calderas. (E) Late Cretaceous. Formation of asthenospheric outer, northeast-dipping subduction zone; slab breakoff and inac- upwelli tive subduction of the inner mid-Cretaceous subduction zone; and ng dextral oblique movement of the continental margin between faults E. Late Cretaceous parallel to the subsequently developed right-lateral Denali and Tin- NORTHEAST tina fault zones (DF, TF). Late Cretaceous plutonism and formation SOUTHWEST DF TF of the Middle Fork caldera. Formation of steep northeast trending sinistral and normal faults (in plane). Abbreviations: Pv—Permian volcanic; Trg—Triassic granite; Jg—Jurassic granite; mKg—mid-Cre- WR mKg lKg KFP mKg taceous granite; lKg—Late Cretaceous granite; KFP—Kula-Farallon plate. Units with v patterns represent rhyolite calderas having ages that correspond to that of the plain color of the unit. fossil slab

In the southeastern Eagle quadrangle, 40Ar/39Ar plateau ages of hornblende Widespread intrusion of mid-Cretaceous calc-alkaline arc granitoids, in- and biotite from Early Jurassic granitoids that intrude the Fortymile River as- cluding the ca. 112–94 Ma dated bodies in our study, occurred throughout the semblage (Table 5) and hornblende, muscovite, and biotite from the Fortymile Yukon-Tanana Upland following mid-Cretaceous regional extension (Fig. 13D) River assemblage and the structurally underlying Nasina assemblage indicate (Wilson et al., 1985; Newberry et al., 1998b). Whole-rock trace element data rapid cooling from ~450 °C to ~350 °C at ca. 188–186 Ma (Hansen et al., 1991; and Pb isotope compositions for feldspars from the mid-Cretaceous intrusions Dusel-Bacon et al., 2002, and references therein). The tectonic origin of the from our study indicate a significant crustal component in the magmas, likely rapid cooling and exhumation is unclear and possible explanations include (1) generated during crustal thickening prior to extension. This conclusion is in gravitational collapse of overthickening crust, (2) uplift via thrusting and ero- accord with Hart et al. (2004) who, on the basis of aeromagnetic signatures, sion (Hansen and Dusel-Bacon, 1998; Dusel-Bacon et al. 1995, 2002; Day et al., magnetic susceptibilities, and whole-rock ferric:ferrous ratios, determined that 2002), and (3) upper plate extension and transtensional strike-slip (sinistral?) 109–102 Ma intrusions in the Yukon-Tanana Upland form an ilmenite-series belt faults as a result of slab rollback (Berman et al., 2007). that developed from melting of continental crust in response to crustal thick- Subsequent oblique northeast-dipping subduction occurred beneath the ening associated with terrane collision. Although some of vein and skarn min- combined allochthonous YTT and parautochthonous YTa and resulted in the eralization in the Mount Harper batholith and elsewhere in east-central Alaska accretion of the Wrangellia composite terrane to the western margin of North occurred within this magmatic episode, the role of these granitoids in gener- America (e.g., Plafker and Berg, 1994) (Fig. 13D). The Wrangellia composite ter- ating the economically significant gold mineralization at the Pogo deposit is rane (also known as the Insular terrane in Canada) is a subcontinental-scale equivocal (Hart et al., 2004, and references therein). Mid-Cretaceous plutonism crustal fragment consisting of the joined Wrangellia, Peninsular, and Alexan- and development of the ca. 108 Ma rhyolite calderas in the Tanacross quad- der terranes that contains Devonian through mid-Cretaceous arcs (Nokleberg rangle (Fig. 2) may have been facilitated by decreasing pressure during exten- et al., 1994). The Wrangellia composite terrane extends from southern Alaska to sion-related exhumation (Dusel-Bacon et al., 1995) (Fig. 13D). British Columbia, was located several thousand kilometers south of its present After formation of the Kula plate at ca. 85 Ma (Plafker and Berg, 1994), position during the Late Triassic, and was translated northward and accreted landward-dipping subduction continued along a new, outer subduction zone to the western margin of North America during Middle Jurassic to Late Creta- (Fig. 13E). Northward motion of the Kula–Farallon–North American plate ceous time. The timing and location for the initial accretion is uncertain owing triple junction resulted in dextral-oblique compression of the continental to differing interpretations of paleomagnetic and geologic data and the history margin and northwestward movement of the now combined allochthonous of postaccretion translation (e.g., Nokleberg et al., 1985; Trop et al., 2002). YTT and parautochthonous YTa along strike-slip faults that likely evolved or In east-central Alaska, 40Ar/39Ar metamorphic cooling ages of ca. 135–110 integrated into the younger right-lateral Tintina and Denali fault systems and Ma are widespread and interpreted to record mid-Cretaceous exhumation of their extensions (Plafker and Berg, 1994). In southern Yukon, ca. 79–72 Ma the lower plate parautochthonous YTa as a result of slab rollback of the sub- small intrusions are spatially associated with, and locally controlled by, ducting plate (Pavlis, 1989; Hansen, 1990; Hansen et al., 1991; Pavlis et al., 1993; the northwest-trending, dextral Big Creek fault that cuts the YTT between Hansen and Dusel-Bacon, 1998) (Fig. 13D). Exhumation occurred during south- and parallel to the Denali and Tintina fault systems (Bennett et al., 2010). In east-vergent (orogen parallel) extensional ductile deformation and metamor- east-central Alaska, subduction is reflected in ca. 75–65 Ma granitoids in the phism documented throughout much of east-central Alaska (Hansen and Du- northwestern Yukon-Tanana Upland (Wilson et al., 1985) and the ca. 70–66 Ma sel-Bacon, 1998). An apparent top-to-the-south-southeast fabric was observed Middle Fork caldera, plutons, and rhyolite dikes along the Kechumstuk fault in tonalitic mylonite gneiss in the Chicken metamorphic complex east of the zone, and the Mount Fairplay intrusion. Kechumstuk fault (Dusel-Bacon et al., 2013) and in a few other locations in the Crustal enrichment is indicated by the 206Pb/204Pb isotopic ratios of feldspars nearby Molly Creek area just west of the Mount Veta ridge (Hansen and Du- from the ca. 70–66 Ma igneous rocks in the western Fortymile area, but their sel-Bacon, 1998). Top-to-the-southeast-vergent deformational fabrics overprint 207Pb/204Pb and 208Pb/204Pb ratios imply that the parental source material of this the earlier top-to-the-northwest fabrics in our study area, and like elsewhere age group contains a smaller upper crustal component and/or a larger mantle in the region, the younger fabrics are interpreted to be generally structurally component than was present in the source of the mid-Cretaceous magmas. shallower than the older fabrics and to have formed during regional exten- The tectonic setting for this mantle input is unknown, but we propose that sion (Hansen and Dusel-Bacon, 1998). An analogous domain of late-Early Cre- after the outer subduction zone of the Kula-Farallon plate was established, the taceous ca. 118–112 Ma exhumation of originally structurally lower rocks has trapped and now inactive (fossil) inner subduction zone was dominated by been identified, based on monazite SHRIMP U-Th-Pb ages and thermobarom- the effects of lithospheric sinking that culminated in slab breakoff and led to etry in amphibolite facies metasedimentary schists, in the Australia Mountain asthenospheric upwelling (Fig. 13E). Such a scenario could explain the Late area in west-central Yukon, and is interpreted to represent a tectonic window Cretaceous juvenile Pb isotopic signatures and peraluminous and metalumi- into a metamorphic core of the YTT, or perhaps the lower plate parautochthonous compositions in our study, the within-plate and syncollisional tectonics nous YTa (Staples et al., 2013). settings indicated for multiple areas of interior Alaska (Newberry, 2000), and

the mantle signature of some volcanic rocks in the Late Cretaceous Carmacks inactive inner subduction zone that delivered the previously accreted Wrangel- Group in Yukon (Johnston et al., 1996). The rapid cooling indicated by the lia composite terrane after the outer Farallon subduction zone was established. agreement of zircon and biotite ages from the Mount Fairplay intrusion indicates that Late Cretaceous magmatism was short lived. Previously proposed tectonic origins of the shoshonitic and high-Mg volcanic rocks and associated ACKNOWLEDGMENTS calc-alkaline intermediate and felsic volcanic rocks of the Carmacks Group in We thank Full Metal Minerals (U.S.A.), Inc., and Full Metal Zinc Ltd., especially Rob McLeod, Vice President of Exploration, for helicopter support and access to drill core and company reports for southwestern Yukon are lithospheric delamination (Mortensen and Hart, 2010) the Mount Veta area. Doyon, Ltd. granted permission required to conduct our studies on Doyon’s and plume-related magmatism (Johnston et al., 1996). selected or conveyed lands in the western Fortymile study area. Charlie Bacon supplied invalu- In east-central Alaska, this dextral-oblique compression of the continental able help in the field, including sample collection and logistics. John Slack and Mike O’Neill are margin and northwestward movement of the crystalline rocks between the also thanked for participation in field work. Discussions with Chris Siron and Cullan Lester of Full Metal Minerals, (U.S.A.), Inc., were especially helpful, and Chris kindly granted permission right-lateral Tintina and Denali fault systems and their precursor faults was key for us to publish the 40Ar/39Ar data. We appreciate the efforts of Joseph Wooden in ensuring that to the far-field development of the northeast-trending sinistral-normal faults the Stanford–U.S Geological Survey instrument worked well during our analytical sessions. We that were associated with Late Cretaceous magmatism and epigenetic miner- thank Renee Pillers for mineral separation work and help with scanning electron microscope imaging of zircons; Janet Gabites, who measured the Pb isotopic compositions of sulfide and alization in the Fortymile district and adjacent areas of Yukon. feldspar samples at the Pacific Centre for Isotopic and Geochemical Research, University of Brit- ish Columbia; and Kate Gans for preparation of final illustrations and many of the rock photos. This manuscript benefited from thorough and insightful reviews by Jamey Jones, Robert Ayuso, Maurice Colpron, Jeff Amato, and Richard Tosdal. Funding for this research was from the U.S. CONCLUSIONS Geological Survey Mineral Resources Program.

New SHRIMP U-Pb zircon ages and published U-Pb and 40Ar/39Ar ages and whole-rock geochemistry of igneous rocks in the Fortymile district define Late REFERENCES CITED Triassic (216–208 Ma), Early Jurassic (199–181 Ma), mid-Cretaceous (112–94 Aleinikoff, J.N., Dusel-Bacon, C., and Foster, H.L., 1981, Geochronologic studies in the Yukon-Ta- Ma), and Late Cretaceous (70–66 Ma) pulses of arc magmatism. The tabular nana Upland, east-central Alaska, in Albert, N.R.D., and Hudson, T., eds., The United States Early Jurassic Mount Veta intrusion and coeval dikes at the LWM prospect are Geological Survey in Alaska: Accomplishments during 1979: U.S. Geological Survey Circular 823-B, p. B34–B37. synkinematic bodies that likely accompanied margin-parallel northwest-di- Aleinikoff, J.N., Dusel-Bacon, C., Foster, H.L., and Nokleberg, W.J., 1987, Pb-isotopic fingerprint- rected crustal shortening during northwest-dipping subduction. ing of tectonostratigraphic terranes, east-central Alaska: Canadian Journal of Earth Sciences, Epigenetic Pb-Zn-Ag ± Cu prospects in the western Fortymile district are v. 24, p. 2089–2098, doi:10.1139/e87-198. Aleinikoff, J.N., Farmer, G.L., Rye, R.O., and Nokleberg, W.J., 2000, Isotopic evidence for the spatially associated with splays of the northeast-trending Kechumstuk sinis- sources of Cretaceous and Tertiary granitic rocks, east-central Alaska—Implications for the tral-normal fault zone and related faults and with ca. 68–66 Ma felsic intrusions tectonic evolution of the Yukon-Tanana terrane: Canadian Journal of Earth Sciences, v. 37, and dikes. Similarity between Pb isotope compositions of feldspars from Late p. 945–956, doi:10.1139/e00-006. Allan, M.M., Mortensen, J.K., Hart, C.J.R., Bailey, L.A., Sanchez, M.G., Ciolkiewicz, W., McKenzie, Cretaceous intrusions and sulfides from the prospects and proximity of pros- G.G., and Creaser, R.A., 2013, Magmatic and metallogenic framework of west-central Yukon pects to the ca. 68–66 Ma bodies suggest a Late Cretaceous age for most of the and east-central Alaska, in Colpron, M., et al., eds., Tectonics, terranes, metallogeny and dis- mineralization. Mineralizing hydrothermal fluids clearly were channeled along covery: The North American Cordillera and similar accretionary settings: Society of Economic Geologists Special Publication 17, p. 111–168. the northeast-trending faults. Intrusion of ca. 68–66 Ma felsic dikes and a stock, Bacon, C.R., and Lanphere, M.A., 1996, Late Cretaceous age of the Middle Fork caldera, Eagle as well as older ca. 95 Ma hypabyssal bodies, within the northeast-trending Ke- quadrangle, east-central Alaska, in Moore, T.E., and Dumoulin, J.A., eds., Geologic studies in chumstuk fault zone was apparently synchronous with recurring sinistral and Alaska by the U.S. Geological Survey, 1994: U.S. Geological Survey Bulletin 2152, p. 143–147, http://pubs.usgs.gov/bul/2152/report.pdf. normal fault displacement. We interpret fault displacements to be a far-field Bacon, C.R., Foster, H.L., and Smith, J.G., 1990, Rhyolitic calderas of the Yukon-Tanana terrane, effect of dextral translation along mid- to Late Cretaceous plate-scale boundar- east-central Alaska—Volcanic remnants of a mid-Cretaceous magmatic arc: Journal of Geo- ies and faults that were roughly parallel to the subsequently developed, largely physical Research, v. 95, no. B13, p. 21,451–21,461, doi:10.1029/JB095iB13p21451. Cenozoic, Denali and Tintina fault systems. Bacon, C.R., Dusel-Bacon, C., Aleinikoff, J.N., and Slack, J.F., 2014, The Late Cretaceous Middle Fork caldera, its resurgent intrusion, and enduring landscape stability in east-central Alaska: The Pb isotopic compositions of feldspars are more radiogenic with de- Geosphere, v. 10, p. 1432–1455, doi:10.1130/GES01037.1 creasing age, consistent with the magmas being mantle derived but exten- Baldwin, S.L., Harrison, T.M., and FitzGerald, J.D., 1990, Diffusion of 40Ar in metamorphic horn- sively contaminated by upper crustal components with evolving Pb isotopic blende: Contributions to Mineralogy and Petrology, v. 105, p. 691–703, doi:10.1007/BF00306534. Bennett, V., Schulze, C., Ouellette, D., and Pollries, B., 2010, Deconstructing complex Au-Ag-Cu compositions. Feldspars from Late Cretaceous igneous rocks have a larger mineralization, Sonora Gulch project, Dawson Range: A Late Cretaceous evolution to the ep- juvenile Pb component relative to compositionally similar rocks of mid-Creta- ithermal environment, in MacFarlane, K.E., et al., eds., Yukon exploration and geology 2009: ceous age, suggesting input of mantle material during generation of Late Cre- Whitehorse, Yukon Geological Survey, p. 23–45. Beranek, L.P., and Mortensen, J.K., 2011, The timing and provenance record of the Late Permian taceous magmas. The tectonic origin of this mantle input is uncertain, but may Klondike orogeny in northwestern Canada and arc-continent collision along western North have been related to asthenospheric upwelling resulting from sinking of the America: Tectonics, v. 30, TC5017, doi:10.1029/2010TC002849.

Berman, R.G., Ryan, J.J., Gordey, S.P., and Villeneuve, M., 2007, Permian to Cretaceous polymeta- Dusel-Bacon, C., Csejtey, B., Jr., Foster, H.L., Doyle, E.O., Nokleberg, W.J., and Plafker, G., 1993, morphic evolution of the Stewart River region, Yukon-Tanana terrane, Yukon, Canada: P-T evo- Distribution, facies, ages, and proposed tectonic associations of regionally metamorphosed lution linked with in situ SHRIMP monazite geochronology: Journal of Metamorphic Geology, rocks in east- and south-central Alaska: U.S. Geological Survey Professional Paper 1497-C, 73 v. 25, p. 803–827, doi:10.1111/j.1525-1314.2007.00729.x. p., scale 1:1,000,000. Breitsprecher, K., and Mortensen, J.K., compilers, 2004, YukonAge 2004—A database of isotopic Dusel-Bacon, C., Hansen, V.L., and Scala, J.A., 1995, High-pressure amphibolite facies dynamic meta- age determinations for rock units from Yukon Territory, Canada: Whitehorse, Yukon Geological morphism and the Mesozoic tectonic evolution of an ancient continental margin, east-central Survey, CD-ROM. Alaska: Journal of Metamorphic Geology, v. 13, p. 9–24, doi:10.1111/j.1525-1314.1995.tb00202.x. Burns, L.E., U.S. Bureau of Land Management, Fugro Airborne Surveys Corp, and Stevens Explo- Dusel-Bacon, C., Lanphere, M.A., Sharp, W.D., Layer, P.W., and Hanson, V.L., 2002, Mesozoic ther- ration Management Corporation, 2008, Line, grid, and vector data, plot files, and descriptive mal history and timing of structural events for the Yukon-Tanana Upland, east-central Alas- project report for the airborne geophysical survey of part of the western Fortymile mining dis- ka—40Ar/39Ar data from metamorphic and plutonic rocks: Canadian Journal of Earth Sciences, trict, east-central Alaska: Alaska Division of Geological and Geophysical Surveys Geophysical v. 39, p. 1013–1051, doi:10.1139/e02-018. Report 2008–1, 9 sheets, scale 1:63,360. Dusel-Bacon, C., Mortensen, J.K., and Fredericksen, R., 2003, Cretaceous epigenetic base-metal min- Cabanis, B., and Lecolle, M., 1989, Le diagramme La/10-Y/15-Nb/8; un outil pour la discrimination eralization at the Lead Creek prospect, eastern Yukon-Tanana Upland, Alaska—Constraints from des series volcaniques et la mise en evidence des processus de melange et/ou de contami- lead isotopic data from sulfides and zircons, in Galloway, J.P., ed., Studies in Alaska by the U.S. nation crustale: Paris, Academie des Sciences Comptes Rendus, v. 309, ser. II, p. 2023–2029. Geological Survey during 2001: U.S. Geological Survey Professional Paper 1678, p. 31–40. Cobb, E.H., 1973, Placer deposits of Alaska: U.S. Geological Survey Bulletin 1374, 213 p. Dusel-Bacon, C., Wooden, J.L., and Hopkins, M.J., 2004, U-Pb zircon and geochemical evidence for Colpron, M., and Ryan, J.J., 2010, Bedrock geology of southwest McQuesten (NTS 115P) and part bimodal mid-Paleozoic magmatism and syngenetic base-metal mineralization in the Yukon-Ta- of northern Carmacks (NTS 115I) map area, in MacFarlane, K.E., et al., eds., Yukon exploration nana terrane, Alaska: Geological Society of America Bulletin, v. 116, p. 989–1015, doi:10.1130 and geology 2009: Whitehorse, Yukon Geological Survey, p. 159–184. /B25342.1. Creaser, R.A., Heaman, L.M., and Erdmer, P., 1997, Timing of high-pressure metamorphism in the Dusel-Bacon, C., Hopkins, M.J., Mortensen, J.K., Dashevsky, S.S., Bressler, J.R., and Day, W.C., Yukon-Tanana terrane, Canadian Cordillera: Constraints from U-Pb zircon dating of eclogite 2006, Paleozoic tectonic and metallogenic evolution of the pericratonic rocks of east-central from the Teslin tectonic zone: Canadian Journal of Earth Sciences, v. 34, p. 709–715, doi: Alaska and adjacent Yukon Territory, in Colpron, M., and Nelson, J.L., eds., Paleozoic evolution 10.1139/e17-057. and metallogeny of pericratonic terranes at the ancient Pacific margin of North America, Ca- Cushing, G.W., 1984, The tectonic evolution of the eastern Yukon-Tanana Upland [M.S. thesis]: Al- nadian and Alaskan Cordillera: Geological Association of Canada Special Paper 45, p. 25–74. bany, State University of New York, 235 p. Dusel-Bacon, C., Slack, J.F., Aleinikoff, J.N., and Mortensen, J.K., 2009, Mesozoic magmatism and Dashevsky, S.S., Nicol, D.L., and Bond, J., 1986, Mines, prospects, and geochemical anomalies on base-metal mineralization in the Fortymile mining district, eastern Alaska—Initial results of Doyon Limited regional overselection lands, Alaska, Blocks 1-8: Doyon, Ltd. Report 86-01a, 300 petrographic, geochemical, and isotopic studies in the Mount Veta area, in Haeussler, P.J., and p. (Report held by Doyon, Ltd., Fairbanks, Alaska). Galloway, J.P., eds., Studies by the U.S. Geological Survey in Alaska, 2007: U.S. Geological Day, W.C., Gamble, B.M., Henning, M.W., and Smith, B.D., 2000, Geologic setting of the Fortymile Survey Professional Paper 1760-A, 42 p., http://pubs.usgs.gov/pp/1760/a/. River area—Polyphase deformational history within part of the eastern Yukon-Tanana Uplands Dusel-Bacon, C., Day, W.C., and Aleinikoff, J.N., 2013, Geochemistry, petrography, and zircon U-Pb of Alaska, in Kelly, K.D., and Gough, L.P., eds., Geologic studies in Alaska by the U.S. Geologi- geochronology of Paleozoic metaigneous rocks in the Mount Veta area of east-central Alaska: cal Survey, 1998: U.S. Geological Survey Professional Paper 1615, p. 65–82. Implications for the evolution of the westernmost part of the Yukon-Tanana terrane: Canadian Day, W.C., Aleinikoff, J.N., and Gamble, B., 2002, Geochemistry and age constraints on metamor- Journal of Earth Sciences, v. 50, p. 826–846, doi:10.1139/cjes-2013-0004. phism and deformation in the Fortymile River area, eastern Yukon-Tanana Upland, Alaska, in Eberhart-Phillips, D., and 28 others, 2003, The 2002 Denali fault earthquake, Alaska: A large magni- Wilson, F.H., and Galloway, J.P., eds., Studies by the U.S. Geological Survey in Alaska, 2000: tude, slip-partitioned event: Science, v. 300, no. 5622, p. 1113–1118, doi:10.1126/science.1082703. U.S. Geological Survey Professional Paper 1662, p. 5–18. Erdmer, P., Ghent, E.D., Archibald, D.A., and Stout, M.Z., 1998, Paleozoic and Mesozoic high-pressure Day, W.C., Aleinikoff, J.N., Roberts, P., Smith, M., Gamble, B.M., Henning, M.W., Gough, L.P., and metamorphism at the margin of ancestral North America in central Yukon: Geological Society Morath, L.C., 2003, Geologic map of the Big Delta B-2 quadrangle, east-central Alaska: U.S. of America Bulletin, v. 110, p. 615–629, doi:10.1130/0016-7606(1998)110<0615:PAMHPM>2.3.CO;2. Geological Survey Geologic Investigations Series I-2788, scale 1:63,360. Foster, H.L., 1970, Reconnaissance geologic map of the Tanacross quadrangle, Alaska: U.S. Geo- Day, W.C., O’Neill, J.M., Aleinikoff, J.N., and Green, G.N., 2007, Geologic map of the Big Delta B-1 logical Survey Miscellaneous Investigations Series Map I-593, scale 1:250,000. quadrangle, east-central Alaska: U.S. Geological Survey Scientific Investigations Map 2975, Foster, H.L., 1976, Geologic map of the Eagle quadrangle, Alaska: U.S. Geological Survey Miscella- scale 1:63,360. neous Investigations Series Map I-922, scale 1:250,000. Day, W.C., O’Neill, J.M., Dusel-Bacon, C., Aleinikoff, J.N., and Siron, C.R., 2014, Geologic map of the Foster, H.L., 1992, Geologic map of the eastern Yukon-Tanana Region, Alaska: U.S. Geological Sur- Kechumstuk fault zone in the Mount Veta area, Fortymile mining district, east-central Alaska: vey Open-File Report 92-313, 26 p., scale 1:500,000. U.S. Geological Survey Scientific Investigations Map 3291, scale 1:63,360, doi:10.3133/sim3291. Foster, H.L., Keith, T.E.C., and Menzie, W.D., 1994, Geology of the Yukon-Tanana area of east-central Dickinson, W.R., 2004, Evolution of the North American Cordillera: Annual Review of Earth and Alaska, in Plafker, G., and Berg, H.C., eds., The geology of Alaska: Boulder, Colorado, Geologi- Planetary Sciences, v. 32, p. 13–45, doi:10.1146/annurev.earth.32.101802.120257. cal Society of America, Geology of North America, v. G-1, p. 205–240. Dilworth, K., Mortensen, J.K., Ebert, S., and Tosdal, R.M., 2007, Cretaceous reduced granitoids in Gabrielse, H., Murphy, D.C., and Mortensen, J.K., 2006, Cretaceous and Cenozoic dextral oro- the Goodpaster mining district, east-central Alaska: Canadian Journal of Earth Sciences, v. 44, gen-parallel displacements, magmatism and paleogeography, north-central Canadian Cordil- p. 1347–1373, doi:10.1139/E07-014. lera, in Haggart, J.W., et al., eds., Paleogeography of the North American Cordillera: Evidence Dodds, C.J., 1992, Denali fault system, in Gabrielse, H., and Yorath, C.J., eds., Structural styles, Ge- for and against large-scale displacements: Geological Association of Canada Special Paper 46, ology of the Cordilleran orogen in Canada: Geological Survey of Canada, Geology of Canada p. 255–276. no. 4, p. 656–657. Godwin, C.I., and Sinclair, A.J., 1982, Average lead isotope growth curves for shale-hosted zinc- Doe, B.R., and Zartman, R.E., 1979, Plumbotectonics, the Phanerozoic, in Barnes, H.L., ed., Geochem- lead deposits, Canadian Cordillera: Economic Geology and the Bulletin of the Society of Eco- istry of hydrothermal ore deposits (second edition): New York, John Wiley and Sons, p. 22–70. nomic Geologists, v. 77, p. 675–690, doi:10.2113/gsecongeo.77.3.675. Dusel-Bacon, C., and Murphy, J.M., 2001, Apatite fission-track evidence of widespread Eocene Goldfarb, R.J., 1997, Metallogenic evolution of Alaska, in Goldfarb, R.J., and Miller, L.D., eds., Min- heating and exhumation in the Yukon-Tanana Upland, interior Alaska: Canadian Journal of eral deposits of Alaska: Economic Geology Monograph 9, p. 4–34. Earth Sciences, v. 38, p. 1191–1204, doi:10.1139/e01-015. Gordey, S.P., and Ryan, J.J., 2005, Geology, Stewart River area (115N, 115-O and part of 115J), Dusel-Bacon, C., and Williams, I.S., 2009, Zircon U-Pb evidence for prolonged mid-Paleozoic plu- Yukon Territory: Geological Survey of Canada Open File 4970, scale 1:250,000. tonism and the ages of crustal sources in east-central Alaska: Canadian Journal of Earth Sci- Hansen, V.L., 1990, Yukon-Tanana terrane; a partial acquittal: Geology, v. 18, p. 365–369, doi:10.1130 ences, v. 46, p. 21–39, doi:10.1139/E09-005. /0091-7613(1990)018<0365:YTTAPA>2.3.CO;2.

Hansen, V.L., and Dusel-Bacon, C., 1998, Structural and kinematic evolution of the Yukon-Tanana Nelson, J.L., and Colpron, M., 2007, Tectonics and metallogeny of the British Columbia, Yukon, and Upland tectonites, east-central Alaska: A record of late Paleozoic to Mesozoic crustal assembly: Alaskan Cordillera, 1.8 Ga to the present, in Goodfellow, W.D., ed., Mineral deposits of Canada: Geological Society of America Bulletin, v. 110, p. 211–230, doi:10.1130/0016-7606(1998)110<0211 A synthesis of major deposit-types, district metallogeny, the evolution of geological prov- :SAKEOT>2.3.CO;2. inces, and exploration methods: Geological Association of Canada Mineral Deposits Division Hansen, V.L., Heizler, M.T., and Harrison, T.M., 1991, Mesozoic thermal evolution of the Yukon-Ta- Special Publication 5, p. 755–791. nana composite terrane—New evidence from 40Ar/39Ar data: Tectonics, v. 10, p. 51–76, doi: Nelson, J., Paradis, S., Christensen, J., and Gabites, J., 2002, Canadian Cordilleran Mississippi 10.1029/90TC01930. valley-type deposits: A case for Devonian–Mississippian back-arc hydrothermal origin: Eco- Harrison, T.M., Duncan, I., and McDougall, I., 1985, Diffusion of 40Ar in biotite: Temperature, pressure nomic Geology and the Bulletin of the Society of Economic Geologists, v. 97, p. 1013–1036, and compositional effects: Geochimica et Cosmochimica Acta, v. 49, p. 2461–2468, doi:10.1016 doi:10.2113 /gsecongeo.97.5.1013. /0016-7037(85)90246-7. Nelson, J.L., Colpron, M., Piercey, S.J., Dusel-Bacon, C., Murphy, D.C., and Roots, C.F., 2006, Paleo- Hart, C.J.R., Goldfarb, R.J., Lewis, L.L., and Mair, J.L., 2004, The northern Cordilleran mid-Creta- zoic tectonic and metallogenetic evolution of pericratonic terranes in Yukon, northern British ceous plutonic province: Ilmenite/magnetite-series granitoids and intrusion-related mineral- Columbia and eastern Alaska, in Colpron, M., and Nelson, J.L., eds., Paleozoic evolution and ization: Resource Geology, v. 54, p. 253–280, doi:10.1111/j.1751-3928.2004.tb00206.x. metallogeny of pericratonic terrane s at the ancient Pacific margin of North America, Canadian Heaman, L., and Parrish, R., 1991, U-Pb geochronology of accessory minerals, in Heaman, L., and and Alaskan Cordillera: Geological Association of Canada Special Paper 45, p. 323–360. Ludden, J.N., eds., Applications of radiometric isotope systems to problems in geology: Min- Nelson, J.L., Colpron, M., and Israel, S., 2013, The Cordillera of British Columbia, Yukon, and eralogical Association of Canada Short Course Handbook, v. 19, p. 59–102. Alaska: Tectonics and metallogeny, in Colpron, M., et al., eds., Tectonics, terranes, metallogeny Johnston, S.T., and Erdmer, P., 1995, Magmatic flow and emplacement foliations in the Early Juras- and discovery: The North American Cordillera and similar accretionary settings: Society of sic Aishihik Batholith, southwest Yukon: Implications for northern Stikinia, in Miller, D.M., and Economic Geologists Special Publication 17, p. 53–109. Busby, C., eds., Jurassic magmatism and tectonics of the North American Cordillera: Geologi- Newberry, R.J., 2000, Mineral deposits and associated Mesozoic and Tertiary igneous rocks within cal Society of America Special Paper 299, p. 65–82, doi:10.1130/SPE299-p65. the interior Alaska and adjacent Yukon portions of the ‘Tintina Gold Belt’: A progress report, in Johnston, S.T., Wynne, P.J., Francis, D., Hart, C.J.R., Enkin, R.J., and Engebretson, D.C., 1996, Yel- Tucker, T.L., and Smith, M.T., eds., The Tintina Gold Belt: Concepts, exploration, and discover- lowstone in Yukon: The Late Cretaceous Carmacks Group: Geology, v. 24, p. 997–1000, doi: ies: British Colombia and Yukon Chamber of Mines Special Volume 2, p. 59–88. 10.1130/0091 -7613(1996)024<0997:YIYTLC>2.3.CO;2. Newberry, R.J., Layer, P.W., Burleigh, R.E., and Solie, D.N., 1998a, New 40Ar/39Ar dates for intru- Lowey, G.W., 1998, A new estimate of the amount of displacement on the Denali fault system sions and mineral prospects in the eastern Yukon-Tanana terrane—Regional patterns and based on the occurrence of carbonate megaboulders in the Dezadeash Formation (Jura-Cre- significance, Alaska, in Gray, J.E., and Riehle, J.R., eds., Geologic studies in Alaska by the taceous), Yukon, and the Nutzotin Mountains sequence (Jura-Cretaceous), Alaska: Bulletin of United States Geological Survey, 1996: U.S. Geological Survey Professional Paper 1595, Canadian Petroleum Geology, v. 46, p. 379–386. p. 131–159. Maniar, P.D., and Piccoli, P.M., 1989, Tectonic discrimination of granitoids: Geological Society of Newberry, R.J., Bundtzen, T.K., Mortensen, J.K., and Weber, F.R., 1998b, Petrology, geochemistry, America Bulletin, v. 101, p. 635–643, doi:10.1130/0016-7606(1989)101<0635:TDOG>2.3.CO;2. age, and significance of two foliated intrusions in the Fairbanks district, Alaska,in Gray, J.E., McCoy, D., Newberry, R.J., Layer, P., DiMarchi, J.J., Bakke, A., Masterman, J.S., and Minehane, D.L., and Riehle, J.R., eds., Geologic studies in Alaska by the United States Geological Survey, 1996: 1997, Plutonic-related gold deposits of interior Alaska, in Goldfarb, R.J., and Miller, L.D., eds., U.S. Geological Survey Professional Paper 1595, p. 117–129. Mineral deposits of Alaska: Economic Geology Monograph 9, p. 191–241. Nokleberg, W.J., Jones, D.L., and Silberling, N.J., 1985, Origin and tectonic evolution of the Ma- McLennan, S.M., 2001, Relationships between the trace element composition of sedimentary rocks claren and Wrangellia terranes, eastern Alaska Range, Alaska: Geological Society of America and upper continental crust: Geochemistry, Geophysics, Geosystems, v. 2, 1021, doi:10.1029 Bulletin, v. 96, p. 1251–1270, doi:10.1130/0016-7606(1985)96<1251:OATEOT>2.0.CO;2. /2000GC000109 Nokleberg, W.J., Plafker, G., and Wilson, F.H., 1994, Geology of south-central Alaska, in Plafker, G., Miyashiro, A., 1974, Volcanic rock series in island arcs and active continental margins: American and Berg, H.C., eds., The geology of Alaska: Boulder, Colorado, Geological Society of America, Journal of Science, v. 274, p. 321–355, doi:10.2475/ajs.274.4.321. Geology of North America, v. G-1, p. 311–365. Mortensen, J.K., 1988, Geology of southwestern Dawson map area, Yukon Territory (NTS 116 B, C): O’Neill, J.M., Day, W.C., Aleinikoff, J.N., and Saltus, R.W., 2010, The Black Mountain tectonic Geological Survey of Canada Open File 1927, scale 1:250,000, doi:10.4095/130455. zone—A reactivated northeast-trending crustal shear zone in the Yukon-Tanana Upland Mortensen, J.K., 1992, Pre-mid-Mesozoic tectonic evolution of the Yukon-Tanana terrane, Yukon of east-central Alaska, in Gough, L.P., and Day, W.C., eds., Recent U.S. Geological Survey and Alaska: Tectonics, v. 11, p. 836–853, doi:10.1029/91TC01169. studies in the Tintina gold province, Alaska, United States, and Yukon, Canada—Results Mortensen, J.K., 1996, Geological compilation maps of the northern Stewart River map area, Klon- of a 5-year project: U.S. Geological Survey Scientific Investigations Report 2007–5289-D, dike and Sixty Mile Districts: Exploration and Geological Services Division, Yukon, Indian and p. D1–D8. Northern Affairs Canada, Open File 1996–1(G), scale 1:50,000. Page, R.A., Plafker, G., and Pulpan, H., 1995, Block rotation in east-central Alaska: A framework for Mortensen, J.K., and Dusel-Bacon, C., 2014, Nature and U-Pb zircon ages of mid-Cretaceous calde- evaluating earthquake potential?: Geology, v. 23, p. 629–632, doi:10.1130/0091-7613(1995)023 ras and tuffs in eastern Alaska and western Yukon: Implications for landscape evolution in the <0629:BRIECA>2.3.CO;2. northern Cordillera: Geological Society of America Abstracts with Programs, v. 46, no. 6, p. 794. Pavlis, T.L., 1989, Middle Cretaceous orogenesis in the northern Cordillera: A Mediterranean an- Mortensen, J.K., and Hart, C.J.R., 2010, Late and post-accretionary magmatism and metallogeny alog of collision-related extensional tectonics: Geology, v. 17, p. 947–950, doi:10.1130/0091 in the northern Cordillera, Yukon and east-central Alaska: Geological Society of America Ab- -7613(1989)017<0947:MCOITN> 2.3.CO;2. stracts with Programs, v. 42, no. 5, p. 676. Pavlis, T.L., Sisson, V.B., Foster, H.L., Nokleberg, W.J., and Plafker, G., 1993, Mid-Cretaceous exten- Mortensen, J.K., Dusel-Bacon, C., Hunt, J., and Gabites, J.E., 2006, Lead isotopic constraints on the sional tectonics of the Yukon-Tanana terrane, Trans-Alaskan Crustal Transect (TACT), east-cen- metallogeny of middle and late Paleozoic syngenetic base metal occurrences in the Yukon-Ta- tral Alaska: Tectonics, v. 12, p. 103–122, doi:10.1029/92TC00860. nana and Slide Mountain/Seventymile terranes and adjacent portions of the North American Pearce, J.A., 1983, The role of sub-continental lithosphere in magma genesis at destructive plate miogeocline, in Colpron, M., and Nelson, J.L., eds., Paleozoic evolution and metallogeny of margins, in Hawkesworth, C.J., and Norry N.J., eds., Continental basalts and mantle xenoliths: pericratonic terranes at the ancient Pacific margin of North America, Canadian and Alaskan Nantwich, United Kingdom, Shiva Publishing Ltd., p. 230–249. Cordillera: Geological Association of Canada Special Paper 45, p. 261–280. Pearce, J.A., and Peate, D.W., 1995, Tectonic implications of the composition of volcanic arc mag- Mortensen, J.K., Hall, B.V., Bissig, T., Friedman, R.M., Danielson, T., Oliver, J., Rhys, D.A., Ross, mas: Annual Review of Earth and Planetary Sciences, v. 23, p. 251–285, doi:10.1146/annurev K.V., and Gabites, J.E., 2008, Age and paleotectonic setting of volcanogenic massive sulfide .ea.23.050195.001343. deposits in the Guerrero terrane of central Mexico: Constraints from U-Pb age and Pb isotopic Pearce, J.A., Harris, N.B.W., and Tindle, A.G., 1984, Trace element discrimination diagrams for the studies: Economic Geology and the Bulletin of the Society of Economic Geologists, v. 103, tectonic interpretation of granitic rocks: Journal of Petrology, v. 25, p. 956–983, doi:10.1093 p. 117–140, doi:10.2113/gsecongeo.103.1.117. /petrology/25.4.956.

Peccerillo, A., and Taylor, S.R., 1976, Geochemistry of Eocene calc-alkaline volcanic rocks from the Stout, J.H., Brady, J.B., Weber, F., and Page, R.A., 1973, Evidence for Quaternary movement on Kastamonu area, northern Turkey: Contributions to Mineralogy and Petrology, v. 58, p. 63–81, the McKinley strand of the Denali fault in the Delta River area, Alaska: Geological Society doi:10.1007/BF00384745. of America Bulletin, v. 84, p. 939–948, doi:10.1130/0016-7606(1973)84<939:EFQMOT>2.0.CO;2. Piercey, S.J., Paradis, S., Murphy, D.C., and Mortensen, J.K., 2001, Geochemistry and paleotectonic Streckeisen, A., 1976, To each plutonic rock its proper name: Earth-Science Reviews, v. 12, p. 1–33, setting of felsic volcanic rocks in the Finlayson Lake volcanic-hosted massive sulphide district, doi:10.1016/0012-8252(76)90052-0. Yukon, Canada: Economic Geology and the Bulletin of the Society of Economic Geologists, Streckeisen, A., and Le Maitre, R.W., 1979, A chemical approach to the modal (QAPF) classification v. 96, p. 1877–1905, doi:10.2113/gsecongeo.96.8.1877. of the igneous rocks: Neues Jahrbuch für Mineralogie Abhandlungen, v. 136, p. 169–206. Piercey, S.J., Nelson, J.L., Colpron, M., Dusel-Bacon, C., Simard, R.-L., and Roots, C.F., 2006, Pa- Sun, S.-s., and McDonough, W.F., 1989, Chemical and isotopic systematics of oceanic basalts: Im- leozoic magmatism and crustal recycling along the ancient Pacific margin of North America: plications for mantle composition and processes, in Saunders, A.D., and Norry, M.J., eds., Geochemical constraints from Yukon-Tanana and related terranes in northern British Colum- Magmatism in the ocean basins: Geological Society of London Special Publication 42, p. bia, Yukon, and eastern Alaska, in Colpron, M., and Nelson, J.L., eds., Paleozoic evolution and 313–345, doi:10.1144/GSL.SP.1989.042.01.19. metallogeny of pericratonic terranes at the ancient Pacific margin of North America, Canadian Szumigala, D.J., Newberry, R.J., Werdon, M.B., Finseth, B.A., Pinney, D.S., and Flynn, R.L., 2000, and Alaskan Cordillera: Geological Association of Canada Special Paper 45, p. 281–322. Major-oxide, minor-oxide, trace-element, and geochemical data from rocks collected in a por- Plafker, G., and Berg, H.C., 1994, An overview of the geology and tectonic evolution of Alaska, in tion of the Fortymile Mining District, Alaska: State of Alaska Division of Geological and Geo- Plafker, G., and Berg, H.C., eds., The geology of Alaska: Boulder, Colorado, Geological Society physical Surveys Raw-Data File 2000-1, 24 p., scale 1:63,360. of America, Geology of North America, v. G-1, p. 989–1021. Tempelman-Kluit, D.J., 1974, Reconnaissance geology of Aishihik Lake, Snag and part of Stewart Rhys, D., DiMarchi, J., Rombach, C., Smith, M., and Friesen, R., 2003, Structural setting, style and River map-areas, west-central Yukon: Geological Survey of Canada Paper 73-41, 93 p. timing of vein-hosted gold mineralization at the Pogo deposit, east-central Alaska: Mineralium Tempelman-Kluit, D.J., 1979, Transported cataclasite, ophiolite and granodiorite in Yukon: Evidence Deposita, v. 38, p. 863–875, doi:10.1007/s00126-003-0372-1. of arc-continent collision: Geological Survey of Canada Paper 79-14, 27 p. Ryan, J.J., and Gordey, S.P., 2002, Bedrock geology of Yukon-Tanana terrane in southern Stewart River Thirlwall, M.F., 2000, Inter-laboratory and other errors in Pb isotope analyses investigated using map area, Yukon Territory: Geological Survey of Canada Current Research no. 2002–A1, 11 p. a 207Pb-204Pb double spike: Chemical Geology, v. 163, p. 299–322, doi:10.1016/S0009-2541 (99) Sanchez, M.G., Allan, M.M., Hart, C.J.R., and Mortensen, J.K., 2013, Orogen-perpendicular mag- 00135-7. netic segmentation of the western Yukon and eastern Alaska Cordilleran hinterland: Implica- Tosdal, R.M., Wooden, J.L., and Rouse, R.M., 1999, Pb isotopes, ore deposits and metallogenic tions for structural control of mineralization, in MacFarlane, K.E., et al., eds., Yukon exploration terranes, in Lambert, D.D., and Ruiz, J., eds., Application of radiogenic isotopes to ore deposits and geology 2012: Whitehorse, Yukon Geological Survey, p. 133–146. and exploration: Society of Economic Geologists Reviews in Economic Geology Volume 12, Scott, D.J., and St-Onge, M.R., 1995, Constraints on Pb closure temperature in titanite based on p. 1–28, doi:10.5382/Rev.12.01. rocks from the Ungava orogen, Canada: Implications for U-Pb geochronology and P-T-t path Trop, J.M., Ridgway, K.D., Manuszak, J.D., and Layer, P., 2002, Mesozoic sedimentary-basin de- determinations: Geology, v. 23, p. 1123–1126, doi:10.1130/0091-7613(1995)023<1123:COPC- velopment on the allochthonous Wrangellia composite terrane, Wrangellia Mountains basin, TI>2.3.CO;2. Alaska: A long-term record of terrane migration and arc construction: Geological Society of Selby, D., Creaser, R.A., Hart, C.J.R., Rombach, C.S., Thompson, J.F.H., Smith, M.T., Bakke, A.A., America Bulletin, v. 114, p. 693–717, doi:10.1130/0016-7606(2002)114<0693:MSBDOT>2.0.CO;2. and Goldfarb, R.J., 2002, Absolute timing of sulfide and gold mineralization: A comparison of Werdon, M.B., Newberry, R.J., and Szumigala, D.J., 2001, Bedrock geologic map of the Eagle A-2 Re-Os molybdenite and Ar-Ar mica methods from the Tintina Gold Belt, Alaska: Geology, v. 30, quadrangle, Fortymile mining district, Alaska: Alaska Division of Geological and Geophysical p. 791–794, doi:10.1130/0091-7613(2002)030<0791:ATOSAG> 2.0.CO;2. Surveys Preliminary Interpretive Report 2001–3b, scale: 1:63,360. Siron, C.R., 2010, Geology of the Little Whiteman carbonate-hosted replacement deposit, western Werdon, M.B., Flynn, R.L., and Szumigala, D.J., 2004a, Alaska resource data file, Eagle quadrangle: Fortymile district, east-central Alaska [M.S. thesis]: Golden, Colorado School of Mines, 123 p. U.S. Geological Survey Open-File Report 2004-1056, 418 p., http://pubs.usgs.gov/of/2004/1056/. Siron, C.R., Hitzman, M.W., and McLeod, R., 2010, Geology of the Little Whiteman carbonate Werdon, M.B., Newberry, R.J., Athey, J.R., and Szumigala, D.J., 2004b, Bedrock geologic map of hosted replacement Zn-Pb-Ag-(Cu) Prospect, western Fortymile district, Alaska, in Goldfarb, the Salcha River–Pogo area, Big Delta quadrangle, Alaska: Alaska Division of Geological and R.J., et al., eds., The challenge of finding new mineral resources: Global metallogeny, inno- Geophysical Surveys Report of Investigations 2004–1B, scale 1:63,360. vative exploration, and new discoveries: Society of Economic Geologists Special Publication Wilson, F.H., Smith, J.G., and Shew, N., 1985, Review of radiometric data from the Yukon crystal- 15, p. 421–436. line terrane, Alaska and Yukon Territory: Canadian Journal of Earth Sciences, v. 22, p. 525–537, Stacey, J.S., and Kramers, J.D., 1975, Approximation of terrestrial lead isotope evolution by a two- doi:10.1139/e85-054. stage model: Earth and Planetary Science Letters, v. 26, p. 207–221, doi:10.1016/0012-821X Wood, D.A., 1980, The application of a Th-Hf-Ta diagram to problems of tectonomagmatic classifi- (75)90088-6. cation and to establishing the nature of crustal contamination of basaltic lavas of the British Staples, R.D., Gibson, H.D., Berman, R.G., Ryan, J.J., and Colpron, M., 2013, A window into the Tertiary volcanic province: Earth and Planetary Science Letters, v. 50, p. 11–30, doi:10.1016/0012 Early to mid-Cretaceous infrastructure of the Yukon-Tanana terrane recorded in multi-stage -821X(80)90116-8. garnet of west-central Yukon, Canada: Journal of Metamorphic Geology, v. 31, p. 729–753, doi: Yeend, W., 1996, Gold placers of the historical Fortymile River region, Alaska: U.S. Geological Sur- 10.1111/jmg.12042. vey Bulletin 2125, 75 p.