Abstract

The Mesozoic magmatic history of the North American margin records the evolution from a more segmented assemblage of parautochthonous and allochthonous terranes to the more cohesive northern Cordilleran orogenic belt. We characterize the setting of magmatism, tectonism, and epigenetic mineralization in the western Fortymile mining district, east-central Alaska, where parautochthonous and allochthonous Paleozoic tectonic assemblages are juxtaposed, using sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon geochronology, whole-rock geochemistry, and feldspar Pb isotopes of Mesozoic intrusions and spatially associated mineral prospects. New SHRIMP U-Pb zircon ages and published U-Pb and 40Ar/39Ar ages indicate four episodes of plutonism in the western Fortymile district: Late Triassic (216–208 Ma), Early Jurassic (199–181 Ma), mid-Cretaceous (112–94 Ma), and Late Cretaceous (70–66 Ma). All age groups have calc-alkalic arc compositions that became more evolved through time. Pb isotope compositions of feldspars from Late Triassic, Early Jurassic, and Late Cretaceous igneous rocks similarly became more radiogenic with time and are consistent with the magmas being mantle derived but extensively contaminated by upper crustal components with evolving Pb isotopic compositions. Feldspar Pb isotopes from mid-Cretaceous rocks have isotopic ratios that indicate magma derivation from upper crustal sources, probably thickened mid-Paleozoic basement. The origin of the mantle component in Late Cretaceous granitoids suggested by Pb isotopic ratios is uncertain, but we propose that it reflects asthenospheric upwelling following slab breakoff and sinking of an inactive inner subduction zone that delivered the previously accreted Wrangellia composite terrane to the North American continental margin, after the outer Farallon subduction zone was established.

Epigenetic Pb-Zn-Ag ± Cu prospects in the western Fortymile district are spatially associated with splays of the northeast-trending Kechumstuk sinistral-normal fault zone and with ca. 68–66 Ma felsic intrusions and dikes. The similarity between Pb isotope compositions of feldspars from the Late Cretaceous igneous bodies and sulfides from the epithermal prospects suggests a Late Cretaceous age for most of the mineralization. Fluid flow along the faults undoubtedly played a major role in mineralization. We interpret displacement on the northeast-trending faults to be a far-field effect of dextral translation along Late Cretaceous plate-scale boundaries and faults that were roughly parallel to the subsequently developed Denali and Tintina fault systems, which currently bound the region.

INTRODUCTION

The Fortymile mining district of east-central Alaska (Cobb, 1973) is located near the juxtaposition of two Paleozoic tectonic assemblages that developed along the northwestern margin of Laurentia, one parautochthonous and the other allochthonous (Fig. 1). The middle to late Paleozoic evolution of these and other related pericratonic assemblages of the northern Cordillera has been well documented (e.g., Dusel-Bacon et al., 2006; Nelson et al., 2006; Piercey et al., 2006). However, fewer studies have addressed the Mesozoic amalgamation to postamalgamation magmatic history of east-central Alaska (Newberry et al., 1998a; Hansen and Dusel-Bacon, 1998; Dusel-Bacon et al., 2002, 2009; Allan et al., 2013). Mesozoic magmatism records the interaction of outboard allochthonous terranes with the inboard amalgamated continental margin and thus helps constrain plate tectonic models of the northern Cordillera. Mesozoic intrusive rocks in the Fortymile mining district are also important because of their spatial association with undated epigenetic mineral prospects. The district is known for its placer gold (Yeend, 1996), and includes dozens of epigenetic base and precious metal prospects (Werdon et al., 2004a; Dusel-Bacon et al., 2003, 2009; Siron et al., 2010; Allan et al., 2013), especially in the western part of the district in the southwestern Eagle 1° × 3° quadrangle (Figs. 2 and 3).

Our study documents magmatic events, structures, and regional tectonics that are important to understanding the metallogeny and mineral potential of this economically important time frame in the Fortymile district, in adjacent Yukon (Allan et al., 2013), and throughout much of Alaska (Goldfarb, 1997). This paper complements a new geological map of the Mount Veta area (by Day et al., 2014), guided by airborne geophysical surveys (Burns et al., 2008), that provides a structural framework for understanding the metallogeny and tectonic evolution of the study area. We present petrologic, whole-rock major, minor, and trace element geochemical data, and zircon U-Pb ages for felsic and intermediate-composition Mesozoic igneous rocks in an ∼2500 km2 area in the western Fortymile mining district and comparable data for a Late Cretaceous alkalic pluton in the eastern part of the district. We also report new Pb isotopic compositions of feldspar from Mesozoic igneous rocks in the study area together with previously published analyses from east-central Alaska to (1) characterize the geochemical evolution of Mesozoic magmatism, and (2) compare the feldspar Pb compositions with both new and previously published sulfide Pb compositions from prospects in the study area to identify the probable causative magmatic episodes for the epigenetic mineralization. Using these data sets, we evaluate and revise previous models for the Mesozoic tectonic and magmatic history and metallogenesis of the Fortymile district (Newberry et al., 1998a; Hansen and Dusel-Bacon, 1998; Dusel-Bacon et al., 2002, 2009; O’Neill et al., 2010) and the northern Cordillera (Allan et al., 2013). Our integrated new explanation for the study area can be tested in other parts of the Yukon-Tanana Upland that have been mapped in less detail and elsewhere in the northern Cordillera. The hydrothermal aspect of the model, specifically the Late Cretaceous phase of mineralization suggested by sulfide and feldspar Pb isotope data from the Fortymile area, can be tested in other parts of the northern Cordillera with direct dating of minerals associated with hydrothermal mineralization or with comparable Pb isotope data for sulfides from mineral prospects and feldspars from associated intrusions.

REGIONAL GEOLOGIC AND TECTONIC FRAMEWORK

The two Paleozoic pericratonic assemblages juxtaposed in the Fortymile mining district are (1) a parautochthonous continental-margin assemblage that contains metamorphosed Devonian–Mississippian rocks, the protoliths of which were granitoids and bimodal within-plate volcanic rocks (Fig. 1), and (2) an allochthonous assemblage that was rifted from the continental margin in Early Mississippian time and contains Mississippian arc- and backarc-related rocks (Fig. 1). To the east in Canada, Pennsylvanian and Permian arc-related metamorphosed intrusive and volcanic rocks (Fig. 1) intrude and overlie the allochthonous continental margin assemblage (e.g., Nelson et al., 2006; Piercey et al., 2006). Following the terminology in Dusel-Bacon and Williams (2009), the continental-margin component, interpreted to have remained inboard of the intervening ocean, is referred to as the parautochthonous 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 Devonian to Early Triassic) ocean basin between the allochthonous YTT and the parautochthonous YTa (e.g., Tempelman-Kluit, 1979; Hansen, 1990; Nelson 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 Mountain–Seventymile terrane (Fig. 1).

Subsequent closure of the intervening ocean basin and northward transport of the allochthonous YTT was caused by dextral translation and southwest-dipping subduction (present-day coordinates) beneath the rifted crustal fragment as recorded in Permian felsic arc volcanic rocks (Klondike assemblage; Fig. 1) (Tempelman-Kluit, 1979; Mortensen, 1992; Erdmer et al., 1998; Creaser et al., 1997; Hansen and Dusel-Bacon, 1998; Nelson et al., 2006). On the basis of detrital zircon data, it was proposed (Beranek and Mortensen, 2011) that the YTT was in proximity to or had overridden the Laurentian margin of northwestern Canada and, hence, that the Slide Mountain ocean was narrow or had closed by Early to Middle Triassic time.

In Alaska, the allochthonous YTT was affected by pre–Late Triassic and Early Jurassic deformation and metamorphism (Hansen and Dusel-Bacon, 1998; Day et al., 2000; Berman et al., 2007; Beranek and Mortensen, 2011) and intrusion of Late Triassic and Early Jurassic granitoids (Fig. 2). Following Early Jurassic contraction, the combined Alaskan continental margin, composed of the upper plate allochthonous YTT and lower plate parautochthonous YTa, was affected by the following events: (1) Middle Jurassic to Late Cretaceous north-dipping subduction and collision of the Wrangellia composite terrane with the Alaskan continental margin (Nokleberg et al., 1985; Plafker and Berg, 1994; Trop et al., 2002); (2) mid-Cretaceous extension followed by intrusion of postkinematic granitoids (Wilson et al., 1985; Newberry et al., 1998b) and formation of rhyolitic calderas (Bacon et al., 1990; Mortensen and Dusel-Bacon, 2014) (Fig. 2); and (3) Late Cretaceous 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 northwestward movement between the right-lateral Tintina and Denali fault systems (Fig. 2) and preexisting parallel faults (Plafker and Berg, 1994).

Geologic evidence from western Canada and eastern Alaska suggests ∼430 km of mostly Eocene dextral displacement across the Tintina fault system (Gabrielse et al., 2006). Most of the ∼370 km dextral displacement on the Denali fault system is thought to be post–Early Cretaceous, much of the movement having occurred in mid-Tertiary time (Dodds, 1992; Lowey, 1998). Northeast-trending faults, with both left-lateral and vertical displacements (Wilson et al., 1985; Foster et al., 1994; Dusel-Bacon and Murphy, 2001; Dusel-Bacon et al., 2009; O’Neill et al., 2010), are mapped or inferred, based on geophysical data, throughout the Yukon-Tanana Upland (Fig. 2). Movement along the northeast-trending faults has been attributed to the clockwise rotation of blocks resulting from the right-lateral movement along the Tintina and Denali faults (e.g., Page et al., 1995; O’Neill et al., 2010).

GEOLOGIC SETTING OF THE WESTERN FORTYMILE DISTRICT

Geology

Mesozoic magmatic rocks and associated epigenetic Pb-Zn-Ag ± Cu prospects in the eastern part of the Yukon-Tanana Upland primarily occur within components of the allochthonous YTT, including the Fortymile River and Nasina 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 assemblage comprises amphibolite facies metasedimentary and metavolcanic rocks, marble, and granodioritic to tonalitic orthogneiss of Early Mississippian age. The Nasina assemblage consists of (1) a sequence of greenschist facies carbonaceous quartzite, phyllite, and schist, (2) marble and greenstone, and (3) minor felsic volcanic rocks of both Mississippian and Permian age. The Chicken metamorphic complex of Mississippian age consists of greenschist facies metavolcanic rocks and subordinate metagabbro, metadiabase, marble, slate, quartz-mica phyllite, and minor quartzite that occur along the eastern and western margins of the Taylor Mountain batholith (Figs. 2 and 3). Similarities in protoliths, arc geochemical signatures of metaigneous rocks, and conodont age ranges of rocks of the Chicken metamorphic complex and the Fortymile River assemblage suggest a shared origin for both units (Dusel-Bacon et al., 2006, 2013), the difference likely being one of metamorphic grade. Mylonitic tonalitic gneiss interlayered with mafic metavolcanic rocks of the Chicken metamorphic complex yielded a sensitive high-resolution ion microprobe (SHRIMP) U-Pb 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 crystallization 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 large batholiths including the ca. 215 Ma pluton of Kechumstuk Mountain in our study area (Fig. 3), the ca. 214 Ma Happy granite of Newberry et al. (1998a), which crops out in the northeastern part of the study area and continues to the northeast into the central Eagle quadrangle, and the ca. 212 Ma Taylor Mountain batholith in the southeastern Eagle quadrangle (Aleinikoff et al., 1981; Cushing, 1984; Werdon et al., 2001) (Fig. 2). The pluton of Diamond Mountain crops out between the pluton of Kechumstuk Mountain and the Taylor Mountain batholith and yields a hornblende 40Ar/39Ar plateau age of 197.3 ± 0.7 (Newberry et al., 1998a) and a U-Pb titanite age of 201.0 ± 1.4 Ma (Dusel-Bacon et al., 2009) that, considered together, indicate a crystallization age of ca. 199 Ma, straddling the boundary between the Late Triassic and Early Jurassic Periods.

A prominent northeast-trending ridge is formed by the ca. 188–180 Ma Early Jurassic intrusion that crops out primarily south of Mount Veta (Cushing, 1984; Dusel-Bacon et al., 2002, 2009; Day et al., 2014) (Figs. 3 and 4), herein referred to as the Mount Veta intrusion. The intrusion was originally designated the syenite of Mount Veta by Foster (1976), but we found no true syenite. Instead, much of the Mount Veta intrusion is composed of coarse-grained, K-feldspar megacrystic, monzonite, quartz monzonite, or quartz monzodiorite. The Mount Veta intrusion and several small apophyses form a northeast-striking elongate body that appears to be a thick, east-dipping sill, the base of which is characterized by a migmatitic zone of Fortymile River assemblage country rock invaded by the Mount Veta intrusion (Day et al., 2014). Ductile tectonic foliation is developed near footwall and roof pendant zones of the Mount Veta intrusion and is parallel to that of the dominant regional penetrative fabrics in the country rock. Titanite from quartz monzonite from the Mount Veta intrusion yielded a concordant thermal ionization mass spectrometry (TIMS) age of 185.4 ± 1.2 Ma; U-Pb systematics in coexisting zircon are complicated by Paleoproterozoic inheritance, but suggest a time of igneous crystallization consistent with the titanite age (Dusel-Bacon et al., 2009). A small hornblende quartz monzonite pluton east of the Mount Veta intrusion and adjacent to the Fish prospect yielded a SHRIMP U-Pb zircon age of 187 ± 3 Ma (29 in Fig. 3) (Dusel-Bacon et al., 2009).

In the western part of the study area, the mid-Cretaceous (ca. 112–105 Ma, i.e., late-Early Cretaceous) Mount Harper batholith (Newberry et al., 1998a; Dusel-Bacon et al., 2002; Day et al., 2007) intruded Devonian augen gneiss and felsic gneiss of the Lake George assemblage (part of the parautochthonous YTa) and schist and quartzite of the Nasina and Fortymile River assemblages of the YTT (Fig. 3). In the northern part of the study area, the ca. 70 Ma Middle Fork caldera (Fig. 3) forms a 10 × 20 km area of rhyolitic welded tuff and granite porphyry (Bacon and Lanphere, 1996; Bacon et al., 2014). Altered rhyolite porphyry collected from drill core at the Fish mineral prospect (Table 1), located ∼12 km east of the Middle Fork caldera, yielded a SHRIMP U-Pb zircon age of 70.5 ± 1.1 Ma (30 in Fig. 3) (Dusel-Bacon et al., 2009).

Faults

Northeast-trending faults and their kinematically related northwest-trending faults are major structural features in the central part of the study area (Burns et al., 2008; Day et al., 2014) (Figs. 3 and 4). The dominant feature is the Kechumstuk fault zone, which occurs in a prominent northeast-trending topographic depression east of the ridge formed by the Mount Veta intrusion (Mount Veta ridge) (Fig. 4). Another northeast-trending fault zone is to the west of the Mount Veta ridge. Northwest- and west-trending faults locally crosscut the northeast-trending faults. The mapped faults are zones of brittle damage that record recurrent fault movement, as evidenced by superimposed offset of preexisting brittle deformation fabrics along fault surfaces, rebrecciated silicic and iron-oxide alteration that was focused within the faults, and brecciation of younger granitic and rhyolitic dikes and small intrusions that were locally emplaced in the fault zones. The northeast-trending faults have much wider damage zones (as wide as 200 m) and longer trace lengths than the narrower (as wide as 10 m) northwest- and west-trending faults.

The Mount Harper batholith is cut by the steep, northeast-trending, Mount Harper fault that accommodated uplift of the Mount Harper block (Newberry et al., 1998a; Dusel-Bacon and Murphy, 2001; Dusel-Bacon et al., 2002; Bacon et al., 2014). The contact between the Lake George and Fortymile River assemblages is not exposed but is interpreted to be a fault based on the tectonic relationship of these assemblages in the eastern part of the Eagle quadrangle (Hansen and Dusel-Bacon, 1998) and on the occurrence of a northwest-trending high-angle fault that we infer continues along Molly Creek and coincides with the contact between these two assemblages (Figs. 3 and 4). The occurrence of a fundamental structural contact between the parautochthonous and allochthonous packages of rocks is suggested by an ∼170 × 400 m tectonic lens of mantle-derived, Alpine-type metaharzburgite that is exposed within a panel of amphibolite in the Fortymile River assemblage that is bounded by northeast-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; Day et al., 2014) (Figs. 3 and 4).

Base and Precious Metal Mineral Prospects

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 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 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 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 a granitic body within 100 m of the Lucky 13 W-rich skarn on Larsen Ridge (prospect A, Fig. 3), interpreted to date vein mineralization within the plug, and skarn mineralization, respectively.

During 2006–2012, intensive mineral exploration for base and precious metals 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 was focused on seven prospects, with most of the drilling and exploration done on the Little Whiteman (LWM) prospect. The LWM and the Mitchell, Little Enchilada, West LWM, and Fish prospects (Table 1) are spatially associated with the northeast-trending Kechumstuk fault zone east of the Mount Veta ridge. The Eva 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-dipping Pb-Zn-Ag ± Cu massive and semimassive sulfide chimneys and mantos that replace marble bodies in the greenschist facies Nasina assemblage. Sulfide replacement occurs in the hanging wall of the northeast-trending, southeast-dipping Kechumstuk fault, adjacent to subsidiary fault splays, and at the contacts of steeply southeast-dipping, sericitically altered felsic porphyry dikes (Fig. 5). Structural interpretations, based on synthesis of dike orientations, observations from LWM drill core, and geologic mapping, indicate both northwest-side-up normal dip-slip and sinistral strike-slip displacement in this region of the Kechumstuk fault (Siron et al., 2010). The largest sulfide replacement bodies are found within ∼25 m of the Kechumstuk fault. An airborne electromagnetic-resistivity-magnetic survey (Burns et al., 2008) and a ground magnetic susceptibility survey, geologic mapping, and drill core logging (Siron, 2010) suggest that the Kechumstuk fault zone has a left-lateral jog that forms a rhomboidal pull-apart releasing bend in the vicinity of the LWM prospect (Siron et al., 2010). Zircon from a quartz diorite sill that overlies the sulfide bodies at the LWM yielded a Late Triassic (210 ± 3 Ma) SHRIMP U-Pb zircon age (Dusel-Bacon et al., 2009) (31 in Fig. 3). Siron et al. (2010) proposed that mineralizing hydrothermal fluids were channeled along the Kechumstuk fault and were vertically restricted by the quartz diorite sill.

The Eva Creek Ag-Zn-Pb-Cu prospect consists of sulfides and supergene minerals in vug fillings, boxworks, and quartz veins within marble (Werdon et al., 2004a; Dusel-Bacon et al., 2009). Metal anomalies in soils at the prospect are elongated in a northeast-southwest direction (Dashevsky et al., 1986), parallel to the faults mapped in the area (Fig. 4). A drill hole penetrated pyrite- and magnetite-bearing biotite schist and calcite-filled breccia zones as much as 17.7 m thick (Dashevsky et al., 1986).

The Drumstick Zn-Pb-Ag prospect, located 2.2 km north of the Eva Creek prospect, consists of thin, spotty mineralized rock in discontinuous marble beds and breccias within a section predominantly composed of quartz-mica 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 marble interlayered with quartzite, gneiss, and schist of the Fortymile River assemblage (Werdon et al., 2004a; Full Metal Minerals, USA, Inc., 2007, in-house report; Dusel-Bacon et al., 2009). The skarns occur in proximity to both the Early Jurassic Mount Veta intrusion and a Late Cretaceous intrusion dated in our study. Mapped structures at the property include prospect-scale folds and northeast- and north-south–trending faults (Full Metal Minerals, USA, Inc., 2007, in-house report). The prospect occurs within 1 km of a northeast-trending normal fault.

RESULTS

Zircon U-Pb Samples and SHRIMP Ages

Zircon U-Pb geochronology was performed using the Standford–U.S. Geological Survey SHRIMP-RG (reverse geometry) on 23 samples of intermediate to felsic igneous rocks from the western Fortymile district in the Eagle quadrangle (Fig. 3) and on three samples from the eastern part of the district in the adjacent Tanacross quadrangle (Fig. 2). The samples were collected from 20 separate plutons, stocks, or dikes. Table 2 presents a summary of the U-Pb geochronology. More details about each sample can be found in Supplemental Files 11 and 22 (Part A, Description of analytical methods; Part B, Figures showing representative cathodoluminescence (CL) images of zircon, concordia plots, and weighted average plots; and Part C, Interpretations of the analytical results), and Supplemental File 33 (SHRIMP U-Th-Pb data). Samples are referred to by 1–28 (map numbers), according to decreasing age, that correspond with map numbers in the text, map figures, and tables. U-Pb zircon age uncertainties are 2σ. Previously published SHRIMP U-Pb zircon ages also are shown in Fig. 3 for map numbers 19 and 20 (Bacon et al., 2014) and 29–31 (Dusel-Bacon et al., 2009).

Late Triassic Intrusive Rocks (216–208 Ma)

All Late Triassic rock samples (1–4, Figs. 6A–6E) are medium-grained, hypidiomorphic granular with a weakly developed foliation defined by alignment of hornblende laths, as well as biotite in samples 2 and 3 (Supplemental File 1 [see footnote 1]). Two of the samples are from large intrusions: sample 2 (215.0 ± 3.5 Ma) is a hornblende-biotite granodiorite from the pluton of Kechumstuk Mountain (Figs. 3 and 4) and sample 3 (212.0 ± 3.3 Ma) is a hornblende-biotite quartz monzodiorite from the Taylor Mountain batholith in the eastern part of the Fortymile district (Fig. 2). The other two Late Triassic samples (1 and 4 in Fig. 3) are from small intrusive bodies: sample 1 (215.5 ± 3.4 Ma) is from a small body of weakly foliated and metamorphosed hornblende quartz diorite (Figs. 3, 4, and 6A) at the base and west of the Mount Veta intrusion and sample 4 (207.9 ± 2.9 Ma) is a foliated hornblende leucotonalite from a small body within the Mount Veta intrusion (Figs. 3, 4, and 6D).

Early Jurassic Intrusive Rocks (191–181 Ma)

Early Jurassic rock samples (5–11, Figs. 6F–6L) vary considerably in texture and mineralogy (Supplemental File 1 [see footnote 1]). Foliation is moderately to well developed in all but one sample (10, Fig. 6K) and is defined by aligned hornblende laths (e.g., Figs. 6H, 6J), K-feldspar megacrysts (e.g., Fig. 6L), and less commonly by biotite books (Fig. 6G), trains of strained and polygonized quartz (e.g., Fig. 6F), and clinopyroxene prisms (Fig. 6I). The Mount Veta intrusion yielded ages of 186.2 ± 3.0, 184.5 ± 3.0, and 181.2 ± 2.6 Ma (Table 2) and forms a northeast-striking elongate, composite body (Fig. 3). Two zircon samples were collected from small, locally deformed granitoids that intrude metasedimentary rocks of the Fortymile River assemblage west of, and at the structural base of, the synkinematic Mount Veta intrusion (Day et al., 2014) (Fig. 4): a foliated leucogranite body (5, Fig. 6E) and a clinopyroxene-bearing granodiorite dike (8, Fig. 5I). Zircon from these two samples yielded ages of 190.5 ± 4.8 and 184.8 ± 2.9 Ma, within the uncertainty of zircon ages from the Mount Veta intrusion.

Two tabular intrusive bodies just east of the Mount Veta intrusion were also dated. Sample 6 is a fine-grained, carbonate- and sericite-altered felsic dike (Fig. 6G) from drill core of the LWM carbonate replacement prospect (Figs. 3 and 5). Based on compositional similarities, we correlate sample 6 with steeply southeast-dipping sheeted felsic porphyry dikes that are concordant with elongate marble bodies. The sheeted dikes crosscut the ca. 210 Ma quartz diorite intrusion and are spatially associated with sulfide replacement in the hanging wall of the northeast-trending, southeast-dipping, sinistral strike-slip and normal dip-slip Kechumstuk fault (Fig. 5). Although all dated zircon grains in sample 6 have similar concentric oscillatory zoning in cathodoluminescence (CL), two distinct age groups were found: one group (n = 5) yielded a weighted average 206Pb/238U age of 187.7 ± 2.3 Ma, whereas the other group (n = 7) yielded a weighted average 206Pb/238U age of 177.9 ± 1.1 Ma. We interpret the older age as the time of crystallization of the dike (see Supplemental File 2, Part A, Fig. F of Part B, and Part C [see footnote 2]) and suggest that the dike was reheated ca. 178 Ma. This interpretation is based on the facts that the dated sample occurs within ∼5 km of a megacrystic phase of the Mount Veta intrusion that is 181 ± 3 Ma (sample 11) and the zircon U-Pb crystallization age of the felsic dike is in agreement with Ar-Ar dating (Supplemental File 44). Incremental heating of secondary sericitic muscovite from another highly altered felsic porphyry dike from a different drill hole yielded a 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 mineral phase (Supplemental File 4 [see footnote 4]).

Sample 10 is a nonfoliated K-feldspar megacrystic hornblende-biotite granite porphyry (Fig. 6K) that forms a thin sill in the hanging wall of the thrust fault that places Chicken metamorphic complex rocks over Nasina assemblage rocks, east of the Mount Veta intrusion and the Kechumstuk fault zone (Figs. 3 and 4). This sample yielded an igneous crystallization age of 183.4 ± 3.6 Ma.

Mid-Cretaceous Intrusive Rocks (112–94 Ma)

Mid-Cretaceous rock samples (12–18, Figs. 7A–7G), in contrast to the Triassic and Jurassic samples, show no evidence of a preferred fabric, are discordant to country rocks, and range from hypidiomorphic granular to porphyritic. We conclude that they were intruded after the regional Early Jurassic dynamothermal metamorphism (Dusel-Bacon et al., 2002). The two oldest samples are leucogranite and intensely altered biotite granite (12 and 13, Figs. 7A and 7B, respectively) from a north-trending, elongate body that intrudes Devonian augen gneiss of the Lake George assemblage in the parautochthonous YTa at the southern edge of the study area (Figs. 3 and 4). The body crops out along a ridge that includes the vertical angle benchmark (VABM) “Corner,” and was designated the Corner granite by Day et al. (2014). Zircon in samples 12 and 13 shows fine oscillatory zoning and yielded crystallization ages of 111.8 ± 1.5 and 108.8 ± 1.7 Ma, respectively.

The next three oldest samples are from the Mount Harper batholith in the western part of the study area (Fig. 3). Sample 14 is a medium-grained, hypidiomorphic granular biotite granite (Fig. 7C) that yielded a crystallization age of 103.2 ± 1.5 Ma based on 11 out of 15 analyses; 4 analyses yielded ages of ca. 113–109 Ma. The other two samples are dikes that cut the Mount Harper batholith at the Section 21 Mo-W porphyry prospect. (1) Sample 15 is a fine-grained aplite dike (Fig. 7D) that contains quartz-molybdenite veins and intrudes a quartz monzonite phase of the Mount Harper batholith. Some of the zircon grains from this sample have extremely high U concentrations (∼1200–8970 ppm) making U-Pb analysis problematic; however, a crystallization age of 101.4 ± 1.4 Ma age was determined for seven analyses of a relatively low U concentration (∼480–1030 ppm) group of zircons. (2) Sample 16, a medium-grained granodioritic quartz porphyry (Fig. 7E), crops out as a linear dike that appears to cut both the quartz monzonite and aplite dike at the Section 21 prospect and yielded a crystallization age of 96.2 ± 1.3 Ma.

Two small porphyry intrusions in the southern part of the Kechumstuk fault zone also yielded zircon U-Pb ages in the younger phase of mid-Cretaceous magmatism: (1) sample 17 (95.8 ± 1.5 Ma) is a quartz-feldspar-biotite rhyolite porphyry (Fig. 7F) that intrudes the Corner granite, and (2) sample 18 (93.9 ± 1.3 Ma) is quartz-plagioclase rhyolite porphyry (Fig. 7G) from a small body that crops out between splays of the Kechumstuk fault zone (Figs. 3 and 4).

Late Cretaceous Intrusive Rocks (68–66 Ma)

Eight Late Cretaceous intrusive rocks dated in this study (samples 21–27, Fig. 8) display no evidence of a preferred fabric and document a relatively brief Late Cretaceous magmatic episode that formed equigranular granitic intrusions and porphyritic rhyolite dikes (Figs. 3 and 4) and, in the Mount Fairplay 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 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 granite (Fig. 8C) from a small intrusion bounded by splays of the Kechumstuk fault zone ∼2 km north of the Mitchell prospect (23, Figs. 3 and 4). Three phases of a composite pluton just southeast of the Middle Fork caldera in an inferred uplifted fault block (Fig. 3) were dated: (1) fine-grained biotite-hornblende granite with dark gray smoky quartz phenocrysts (22, Figs. 3 and 8B; Granite of Gold Bottom Creek unit of Day et al., 2014) is 67.9 ± 1.1 Ma, (2) biotite-hornblende granite is 65.8 ± 1.5 Ma (26, Figs. 3 and 8G), and (3) biotite granite is 65.8 ± 1.4 Ma (27, Figs. 3 and 8H; Granite of Veta Creek unit of Day et al., 2014).

Zircon from a microporphyritic aplite dike (sample 28, Figs. 3 and 8I) that occurs within a splay of the Kechumstuk fault zone that cuts the pluton of Kechumstuk Mountain (Fig. 4) appears to be mostly (or possibly entirely) xenocrystic. Zircon grains vary in color, external morphology, and CL zoning patterns. The 206Pb/238U ages range from ca. 93 to 64 Ma; the ca. 65 Ma age of the two youngest grains is interpreted to represent the maximum age of emplacement.

Late Cretaceous ages also were determined for two samples from the syenite of Mount Fairplay unit in the central Tanacross quadrangle (Foster, 1970) (Fig. 2). Clinopyroxene-hornblende-biotite syenite (24) is characterized by distinctive gray, aligned, 2–6-cm-long, tabular megacrysts of K-feldspar that make up ∼30% of the rock (Figs. 8D, 8E). Zircon is medium to dark brown, anhedral to subhedral, and yielded a crystallization age of 67.0 ± 1.5 Ma. CL imagery and scanning electron microscopy–energy dispersive spectroscopy analysis reveal that most grains are composed of relict, oscillatory zoned baddeleyite (ZrO2) that was partially replaced by zircon (Supplemental File 2, Part B, Fig. V [see footnote 2]). A sample of medium-grained, equigranular hornblende-biotite quartz monzonite (25, Fig. 8F) from a different phase of the syenite of Mount Fairplay unit gave a crystallization age of 66.5 ± 1.1 Ma for colorless, euhedral zircon (Supplemental File 2, Part B, Fig. W [see footnote 2]), which overlaps within analytical uncertainty the age for sample 24.

Whole-Rock Geochemistry

Table 3 provides whole-rock geochemical analyses for Fortymile district Mesozoic felsic and intermediate-composition granitoids dated by U-Pb geochronology in this study and associated, previously dated or undated, igneous rocks of clear age designation, together with three Mesozoic mafic igneous rocks from the western Fortymile district. Location information for whole-rock samples is given in Supplemental File 55. All samples were analyzed by wavelength-dispersive X-ray fluorescence and inductively coupled plasma–mass spectrometry at the GeoAnalytical Laboratory, Washington State University (see Dusel-Bacon et al., 2013, for a detailed description of analytical procedures).

Felsic and Intermediate-Composition Rocks

Whole-rock geochemical plots of the Mesozoic felsic and intermediate-composition samples analyzed in our study, together with analyses of dated Mesozoic igneous rocks from the Fortymile district (reported in Dusel-Bacon et al., 2009), are shown in Figure 9, grouped by age. Our petrographic observations of sample mineralogy are consistent with the rock names based on volume percent quartz (Q), orthoclase (Or), and albite (Ab) + anorthite (An) (Fig. 9A) calculated from whole-rock analyses using the weight-percent normalization scheme of Streckeisen and Le Maitre (1979). Late Triassic igneous rocks consist of quartz monzodiorite or granodiorite and/or tonalite. Early Jurassic rocks display a wide range of silica-undersaturated to silica-saturated compositions (monzonite, quartz monzonite, quartz monzodiorite, granodiorite and/or tonalite, and granite), whereas mid- and Late Cretaceous rocks have more restricted, quartz-rich compositions of granite and granodiorite (Fig. 9A). On a K2O-SiO2 diagram (Fig. 9B), most Late Triassic samples plot in the calc-alkaline series and Early Jurassic samples span a range from calc-alkaline to high-K-calc-alkaline to shoshonitic series; both age groups show a wide variability in silica content (SiO2 range of ∼52–72 wt%). Mid- and Late Cretaceous igneous samples are more chemically restricted and plot within the high-K calc-alkaline series, with the exception of the 67 Ma syenite from Mount Fairplay, which plots within the shoshonitic series. A plot of Al/(Na + K) versus Al/(Ca + Na + K) (Fig. 9C) classifies most Late Triassic and Early Jurassic samples as metaluminous and most Cretaceous samples as either peraluminous or straddling the boundary between these two fields; the 67 Ma syenite and quartz monzonite samples from the Mount Fairplay intrusion plot in the metaluminous field. All of the samples follow a calc-alkalic compositional trend shown by decreasing FeO*/MgO with decreasing TiO2 (Fig. 9D). In the Ta versus Yb tectonic-discrimination diagram (Fig. 9E), almost all samples plot in the volcanic-arc granite field. Late Cretaceous samples have slightly higher Ta concentrations that cause them to plot either just inside the arc field close to its border with 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 in the field of within-plate and anomalous ocean ridge-type granite (Fig. 9E).

Primitive mantle-normalized multielement plots for dated samples in each of the six Mesozoic age groups are shown in Figures 9F–9K. All of the Fortymile igneous samples have low normalized abundances of Nb and Ta relative to Th and La, a hallmark of modern arc rocks (Sun and McDonough, 1989). Negative Eu and Ti anomalies record the removal of previously crystallized plagioclase and Fe-Ti oxides, respectively, from the melt. Trace element patterns of the four Mesozoic groups show progressively more evolved compositions through time, indicated by increasing concentrations of Th and progressively more negative slopes of the adjacent incompatible elements to the left of the 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 batholith have multielement patterns that overlap those of the Late Triassic rocks in the Mount Veta area. Multielement plots of the dated Early Jurassic rocks in the Mount Veta area are comparable to those in the eastern Fortymile area and all have parallel patterns with a moderate range in overall primitive mantle-normalized trace element contents (Fig. 9G).

The mid-Cretaceous (Figs. 9H, 9I) and Late Cretaceous (Figs. 9J, 9K) samples are the most evolved intrusive rocks. Four compositionally evolved mid-Cretaceous granitoid samples differ from the others in these age groups in more pronounced negative Eu and Ti anomalies (Fig. 9H): these are the 101 Ma aplite from the dike at the Section 21 prospect in the Mount Harper batholith and one dated (112 Ma leucogranite) and two undated samples (quartz feldspar porphyry dike sample 08AD054 and sample 09AD311) from the Corner granite intrusion. The leucogranite has the highest primitive mantle-normalized heavy rare earth element (REE) contents. Sample 09AD-311, collected 1.5 km north of the 109 Ma sample 13 (Fig. 3), is an altered biotite granite with smoky quartz that contains ubiquitous granophyric quartz-alkali feldspar intergrowths that have a herringbone-like texture (Figs. 7H, 7I) indicative of late eutectoid crystallization.

Six Late Cretaceous samples from the ca. 70 Ma Middle Fork caldera have identical multielement patterns (Fig. 9J), similar to those of most of the mid-Cretaceous rocks. The ca. 68–66 Ma Late Cretaceous samples from the intrusion north of Mount Veta, the felsic dikes and small intrusions associated with the Kechumstuk fault zone, and Mount Fairplay have multielement patterns similar to those from the mid-Cretaceous and ca. 70 Ma samples, but show a wider range of trace element contents (Fig. 9K). Syenite from the Mount Fairplay intrusion (24, Fig. 2) has the highest trace element contents and the felsic dike sample (21, Fig 3) from within the Kechumstuk fault zone has the lowest trace element contents. Microporphyritic aplite dike sample (28), the U-Pb zircon age of which we interpret to be ca. 65 Ma or younger, plots below all the ca. 68–66 Ma samples.

Mafic Rocks

Three samples of mafic igneous rocks were collected for whole-rock geochemical analyses in order to compare their trace element signatures with 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 geochemical signature as a result of generation in, or contamination by, continental crust; (2) felsic magmas can represent blending of partial melt contributions from many different continental lithologies (e.g., Piercey et al., 2001); and (3) the abundances of some high field strength elements (HFSEs) (including Ti, Zr, and Hf) and REEs are extremely sensitive to accessory mineral fractionation and removal from the melt.

The first mafic igneous rock sample is Mesozoic diorite, collected ∼200 m south of the dated Late Triassic hornblende-biotite granodiorite from the pluton of Kechumstuk Mountain (2, Fig. 3) near the Mitchell prospect; it is weakly foliated, fine grained, and composed of hornblende and plagioclase. The relationship of the diorite to the Triassic intrusion is unknown. The presence of foliation in the diorite suggests a pre-Cretaceous age, and we interpret the diorite to be either a cognate phase of the Late Triassic intrusion or a Jurassic dike related to the Mount Veta intrusion. The second sample is a nonfoliated diorite dike that cuts the ca. 110 Ma Corner granite. The diorite is medium grained and contains approximately equal amounts of hornblende and highly altered plagioclase. The absence of a preferred fabric in the dike suggests a mid-Cretaceous or younger age; because Tertiary dikes, where mapped in the adjacent Big Delta quadrangle to the west (Day et al., 2007), are generally unaltered, we consider a Tertiary age less likely. The third sample is a rare magmatic enclave within the ca. 70 Ma granite porphyry of the Middle Fork caldera. The 6 × 8 × 12 cm mafic enclave is a fine-grained hornblende-plagioclase-biotite diorite with a texture that shows that the enclave magma crystallized rapidly in undercooled conditions. Bacon et al. (2014) suggested that the enclave may be similar to parental magma of the caldera and indicates an input of higher temperature mafic magma.

Whole-rock geochemical analyses of the mafic rocks plot in the calc-alkalic arc field on immobile trace element diagrams utilizing HFSEs (Figs. 10A, 10B). A plot of Nb/Yb versus Th/Yb (Fig. 10C) has been shown to be relatively unaffected by partial melting and fractional crystallization and to reflect mantle sources of basalt (Pearce, 1983; Pearce and Peate, 1995). Global averages or typical values for characteristic magma settings, as well as trends for within-plate enrichment, crustal contamination, and subduction-zone enrichment (slab metasomatism), are shown for comparative purposes in Figure 10C. Multielement patterns (Fig. 10D) exhibit negative Nb and Ta anomalies relative to Th and La, a geochemical characteristic that is typical of modern arcs.

The Cretaceous dike and especially the mafic enclave show geochemical evidence of crustal contamination in elevated Th contents, which cause them to (1) plot very close to the composition of average upper crust (Fig. 10A) within the calc-alkalic end of the volcanic arc basalt field; (2) follow the trend of crustal contamination or subduction zone enrichment in the Nb/Yb versus Th/Yb diagram (Fig. 10C); and (3) have highly developed Nb-Ta troughs and negatively sloping multielement patterns (Fig. 10D). Thus, tectonic signatures based on trace element concentrations of the mafic rocks are consistent with those indicated for the Cretaceous felsic and intermediate-composition igneous rocks.

Pb Isotope Data

In Table 4 we present 15 new analyses of Pb isotopic compositions of feldspar separates from dated igneous rocks in the western Fortymile district, as well as an analysis of feldspar from the Late Triassic granodiorite of the Taylor Mountain batholith, east of the study area. We also report new and previously published Pb isotopic compositions of sulfides from the LWM, Eva, Drumstick, and Oscar Pb-Zn-Ag-Cu prospects in the western Fortymile district. Sample locations are shown in Figure 11, with the exception of that for feldspar from Taylor Mountain, which is shown in Figure 2.

Pb isotopic compositions of feldspars from igneous rocks provide a means of identifying the contributions of mantle and crustal Pb in parental magmas and characterizing magmatism both spatially and temporally. Figure 12A shows the Pb isotopic ratios of the 15 igneous feldspars determined in our study. Figure 12B presents the isotopic compositions of 30 previously published analyses of igneous feldspars collected in east-central Alaska (Aleinikoff et al., 1987, 2000; Dilworth et al., 2007) in relation to the fields defined by the igneous feldspar compositions determined in our study. The feldspar Pb isotopic compositions are divided into the following age groups, based on the U-Pb zircon or titanite crystallization age of the sample: Late Triassic to earliest Jurassic (pluton of Diamond Mountain) (215–199 Ma), Early Jurassic (187–181 Ma), mid-Cretaceous (115–95 Ma) and Late Cretaceous (94–78, 73–70, and 68–66 Ma) (Fig. 12.) All of the samples in the 94–78 Ma age group and all but 2 of the 18 samples in the 115–95 Ma age group (Fig. 12B) are from outside the study area, in other parts of the Yukon-Tanana Upland or the foothills to the Alaska Range south of the Tanana River (Aleinikoff et al., 2000; Dilworth et al., 2007). The shale curve of Godwin and Sinclair (1982), which closely approximates the Pb isotopic evolution of upper crustal rocks in the northern Cordillera (including both the parautochthonous YTa and the allochthonous YTT; Mortensen et al., 2006), the average crustal growth curve of Stacey and Kramers (1975), and the estimated average mantle curve of Doe and Zartman (1979) are shown for reference in Figure 12, as is the field of feldspar data derived from Pb isotopic compositions of Late Devonian and Early Mississippian metaigneous rocks of the parautochthonous YTa (Aleinikoff et al., 1987).

Feldspar from rocks of Late Triassic–earliest Jurassic, Early Jurassic, 94–78 Ma early-Late Cretaceous, and others of Late Cretaceous age, in our study and other studies have 206Pb/204Pb ratios that range from 18.724 to 19.454 and define fields that become more radiogenic with time. On 206Pb/204Pb versus 207Pb/204Pb diagrams (Figs. 12A, 12B), these four age groups plot below the shale curve of Godwin and Sinclair (1982) and are significantly more radiogenic than values for the average crustal growth curve of Stacey and Kramers (1975). In contrast, feldspar Pb isotopes from 115–95 Ma mid-Cretaceous rocks from our study (n = 2) plot above the shale curve and within the range of previously analyzed mid-Cretaceous granitic rocks of this age from adjacent parts of east-central Alaska (Fig. 12B). The 115–95 Ma feldspars overlap the less radiogenic part of the field of feldspar data from the Late Devonian and Early Mississippian metaigneous rocks (Figs. 12A, 12B). The isotopic ratios for Devonian and Mississippian rocks plot above the shale curve because these rocks contain significant components of Archean and Proterozoic upper crustal material, as shown by zircon core inheritance ages and detrital zircon ages of 3.3–1.1 Ga (Dusel-Bacon and Williams, 2009). The one exception to Pb isotopic age grouping is the anomalously radiogenic composition of feldspar from the 68 Ma sample (23) that plots within the area of 115–95 Ma mid-Cretaceous rocks (Fig. 12A).

Similar grouping of feldspar isotopic compositions by intrusive age is evident in the 206Pb/204Pb versus 208Pb/204Pb plot (Fig. 12A); 208Pb/204Pb ratios are positively correlated with increasing 206Pb/204Pb, indicating an increasing radiogenic component with decreasing age. Mid-Cretaceous (115–95 Ma) feldspars yielded the most radiogenic ratios, similar to feldspars from Late Devonian and Early Mississippian metaigneous rocks.

Pb isotopic compositions of 8 galena samples, 1 sphalerite sample, and 1 pyrite sample (including 5 new analyses and 5 reported in Dusel-Bacon et al., 2009) from 4 Pb-Zn-Ag prospects in the western Fortymile district have 206Pb/204Pb ratios of 19.301–19.521, 207Pb/204Pb ratios of 15.645–15.725, and 208Pb/204Pb ratios of 39.106–39.357 (Table 4). Pb isotopic compositions of sulfides in intrusion-related mineralization, such as sulfides from proximal skarns, porphyry-type deposits, or more distal veins, are commonly similar to those of igneous feldspars in genetically related intrusions, assuming closed-system behavior (Tosdal et al., 1999).

The Pb isotopic compositions of sulfide minerals from the LWM, Eva Creek, and Oscar prospects (Fig. 12C) plot below the shale curve and overlap the field of feldspar Pb compositions of samples from the ca. 70 Ma Middle Fork caldera, the ca. 66 Ma intrusion north of Mount Veta, and a feldspar from a ca. 72 Ma felsic porphyry at the Bluff Cu-Mo prospect in the eastern Tanacross quadrangle (Mortensen, unpub. data; Allan et al., 2013) (Fig. 12A). Pb isotopic compositions of two galena samples from the Drumstick prospect have slightly higher 207Pb/204Pb values and plot near the feldspar compositions of a sample from the compositionally evolved ca. 110 Ma Corner granite and a sample from the ca. 66 Ma granite near the West LWM prospect (Figs. 12A, 12C).

DISCUSSION

Magmatism Viewed in Regional Context

Table 5 shows the U-Pb ages from our study (gray highlighting) together with previously determined radiometric ages of Mesozoic igneous rocks from various parts of the Eagle quadrangle, as well as a few relevant ages from the adjacent Big Delta and Tanacross quadrangles. The following closure temperatures are used in the discussion of the qualitative evaluation of cooling rates: ≥900 °C, U-Pb closure temperature of zircon (Heaman and Parrish, 1991): 660–700 °C, U-Pb closure temperature of titanite (Scott and St-Onge, 1995); 450 ± 50 °C, 40Ar/39Ar closure temperature of hornblende (Baldwin et al., 1990); and 300 ± 50 °C, 40Ar/39Ar closure temperature for biotite (Harrison et al., 1985).

Late Triassic U-Pb SHRIMP ages indicate magmatic episodes ca. 215–212 and ca. 210–208 Ma (Table 5). Zircon and titanite U-Pb ages from different parts of the Kechumstuk pluton are within analytical uncertainty, suggesting relatively rapid cooling from ≥900 °C at 215.5 ± 3.4 Ma through 660–700 °C at 215.7 ± 3.1 Ma. A 207.9 ± 2.9 Ma SHRIMP age for leucotonalite west of the Kechumstuk pluton is within analytical uncertainty of 40Ar/39Ar ages of 207.8 ± 1.2 and 205.6 ± 1.0 Ma for biotite and hornblende, respectively, from the same sample (A in Table 5) from the Kechumstuk pluton collected ∼10 km south of the sample that yielded the 215.5 ± 3.4 Ma U-Pb age. It is not known whether the 40Ar/39Ar ages indicate rapid cooling from 450 ± 50 °C to 300 ± 50 °C of the younger phase of the pluton at 207 Ma or slow cooling from ca. 215 Ma. Geochronology from the Taylor Mountain batholith in the southeastern Eagle quadrangle suggests relatively rapid cooling from ≥900 °C to 660–700 °C at 212 Ma for one sample, rapid cooling from ∼450 °C to 300 °C at 211–209 Ma for another, and slow cooling from ∼450 °C to 300 °C at 209–204 Ma for a third sample (samples C, D, and E in Table 5).

Intrusion of the pluton of Diamond Mountain in the earliest Jurassic (ca. 199 Ma) was followed by the intrusion of Early Jurassic granitoids in the western Fortymile district; SHRIMP U-Pb ages range from 190.5 ± 4.8 Ma for the small leucogranite body west of and at the base of the Mount Veta intrusion to 181.2 ± 2.6 Ma for K-feldspar megacrystic hornblende quartz monzonite from the northeastern part of the intrusion (Fig. 4; Table 5). The other dated megacrystic Mount Veta sample, collected from the southern part of the intrusion, yielded an age of 186.2 ± 3 Ma; thus, given the analytical uncertainties of the two ages, it is not possible to determine the temporal relationship between the megacrystic phases of the intrusion and the other textural variants within it. Hornblende from the same sample that yielded the 185.4 ± 1.2 Ma titanite U-Pb age gave an integrated 40Ar/39Ar age of 179.1 ± 1.1 Ma (B in Table 5), either requiring ∼8.6–4.0 m.y. (depending on uncertainties) to cool from 660–700 °C to ∼450 °C, or reheating of the dated sample by the younger phases of the intrusion, such as that that yielded the ca. 181 Ma U-Pb age. Hornblende from 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 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 (Fig. 2; Table 5).

These Early Jurassic U-Pb zircon ages of plutonic rocks from the southwestern Eagle quadrangle are within the range of ca. 197–185 Ma radiometric ages of most of the Early Jurassic intrusions in the southeastern Eagle quadrangle, as well as quartz monzonite that forms the ca. 183 Ma Seventymile pluton in the northern Eagle quadrangle (Table 5). Biotite and hornblende from the same from the Seventymile pluton yielded closely constrained, overlapping ages (I in Table 5), indicating rapid cooling from ∼450 to 350 °C at 183 Ma. Zircon, titanite, and hornblende ages from the Napoleon Creek pluton indicate rapid cooling from >900 °C to 660–700 °C (H in Table 5) at 188 Ma, with another sample cooling through ∼450 °C at 187 Ma (Table 5). Crystallization ages by different methods and from different locations in the Pig and Mount Warbelow plutons vary considerably (Table 5) and likely indicate either unrecognized composite phases to the plutons or complexities in mineral cooling histories. Feldspars from Early Jurassic rocks have Pb isotopic ratios intermediate between those of the Late Triassic and earliest Jurassic rocks and the Cretaceous rocks (Fig. 12), indicating a smaller ratio of mantle to crustal source material than was present during generation of Late Triassic magmas.

Two phases of the mid-Cretaceous magmatism that postdated the Early Jurassic and Early Cretaceous dynamothermal metamorphic episodes that affected the Yukon-Tanana Upland (Hansen and Dusel-Bacon, 1998; Dusel-Bacon et al., 2002) are present southwest of the Mount Veta ridge and in the Mount Harper area to the west (Fig. 3; Table 5). The older phase (ca. 112–101 Ma) is represented by the Corner granite and Mount Harper batholith and dikes intrusive 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 and biotite 40Ar/39Ar ages from different localities within the Mount Harper batholith range from 110.5 ± 1.1 to 103.2 ± 1.5 Ma; late-stage mineralized quartz veins and aplite dikes formed at 102.7 ± 0.4 and in 101.4 ± 1.4 Ma, respectively. The Mount Harper batholith is probably composite in nature (it has only been mapped in a reconnaissance manner) and it is therefore not possible to know whether different ages reflect different pulses of magmatism or differences in mineral cooling ages or other aspects of the geochronology. Granitoids of this age make up the Walker Fork pluton in the southeastern Eagle quadrangle and Ruby Creek granite in the central Eagle quadrangle (Table 5) and are present in the southeastern Big Delta quadrangle to the west and adjacent Yukon (Day et al., 2003; Dilworth et al., 2007; Werdon et al., 2004b; Hart et al., 2004, and references therein) (Fig. 2). The extrusive equivalent of this earlier mid-Cretaceous magmatic episode is preserved in rhyolite calderas in the Tanacross quadrangle (Bacon et al., 1990) (Fig. 2) that yield preliminary laser ablation U-Pb zircon ages of ca. 110 and 108 Ma for welded tuff (Mortensen and Dusel-Bacon, 2014).

The ca. 96–94 Ma phase of early-Late Cretaceous magmatism is represented in the southwestern Eagle quadrangle by a small intrusion and quartz feldspar porphyry dikes that crosscut the intrusions or country rock in or near the southern part of the Kechumstuk fault zone and dikes in the Mount Harper batholith (Figs. 3 and 4) and the ca. 93 Ma Upper Granite Creek intrusion in the northern Eagle quadrangle (Table 5). A northeast-trending quartz feldspar 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 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 Cretaceous age group, whole-rock geochemistry of the ca. 96–94 Ma rocks indicates granitic, high-K calc-alkalic, and peraluminous compositions and an arc origin, but their 207Pb/204Pb and 208Pb/204Pb isotopic ratios are slightly lower than those in the 115–95 Ma rocks, implying a smaller crustal component in the younger Cretaceous magmas.

Late Cretaceous felsic magmatism in the Fortymile district produced the ca. 70 Ma intracaldera tuff, granite porphyry, and outflow tuff of the Middle Fork caldera (Bacon et al., 2014) and altered rhyolite porphyry intersected in drill core from the Fish prospect (Dusel-Bacon et al., 2009) (Table 5). These ages are within analytical uncertainty of our SHRIMP U-Pb zircon crystallization ages for the intrusion north of Mount Veta, the intrusion and dike associated with the Kechumstuk fault, and the younger phase of the intrusion north of Mount Veta (Table 5). Zircon ages of 67.0 ± 1.5 and 66.5 ± 1.1 Ma from the Mount Fairplay intrusion in the southern Fortymile district in the Tanacross quadrangle (24 and 25 in Fig. 2) are within analytical uncertainty of the ca. 70–66 Ma ages for felsic magmatism in the Mount Veta area. Quartz monzonite from the Mount Fairplay intrusion yielded a biotite K-Ar age of 67 ± 2 Ma (Wilson et al., 1985), indicating rapid cooling from >900 °C to ∼350 °C at ca. 67 Ma (Table 5). Eruption of volcanic rocks of the Carmacks Group and intrusion of the small, high-level, 72–67 Ma Prospector Mountain plutonic suite in southwestern Yukon, Canada (Breitsprecher and Mortensen, 2004; Gordey and Ryan, 2005; Allan et al., 2013), were roughly contemporaneous with Late Cretaceous magmatism in the Fortymile district.

The ca. 65 Ma or younger microporphyritic aplite dike from in the southern Kechumstuk fault zone has an arc geochemical signature comparable to that of the Late Cretaceous felsic rocks in the western Fortymile area. We therefore consider it more likely related to the end stage of that magmatic episode than to subsequent Paleocene and Eocene (ca. 60–50 Ma within-plate magmatism that produced small volumes of comagmatic felsic plutons and mafic dikes, bimodal mafic-felsic volcanic rocks, and shallow intrusions that are present sporadically in much of interior Alaska and Yukon (Bacon et al., 1990; Foster et al., 1994; Newberry, 2000).

Elsewhere in the northern Cordillera, Late Cretaceous igneous rocks display a variety of non-arc geochemical compositions and tectonic signatures. Newberry (2000) showed that Late Cretaceous plutonic rocks from interior Alaska outside of the western Fortymile area include both a 78–60 Ma peraluminous group with syncollisional tectonic signatures and a 71–68 Ma metaluminous group with within-plate tectonic signatures. In southwestern Yukon, volcanic rocks of the Carmacks Group exhibit an even broader compositional variation that includes K- and Mg-rich (shoshonitic) basalt and basaltic andesite (Johnston et al., 1996) and lesser calc-alkalic intermediate and felsic volcanic rocks (Tempelman-Kluit, 1974; Ryan and Gordey, 2002; Colpron and Ryan, 2010, and references therein).

Age of Epigenetic Mineralization in the Western Fortymile District

Base and precious metal epigenetic prospects in the Mount Veta area are spatially associated with both Early Jurassic and mid- and Late Cretaceous intrusions and dikes (Figs. 3 and 4). Although we did not determine the time of mineralization directly, a comparison of Pb isotopic data for igneous feldspars from dated intrusions and prospect sulfides indicates that mineralization probably was associated with Late Cretaceous magmatism that produced the ca. 70 Ma Middle Fork caldera or the 68–66 Ma granitic intrusions and dikes. Galena Pb isotopic compositions from the Drumstick prospect also overlap feldspar Pb ratios from one of the samples from the ca. 110 Ma Corner granite, allowing the possibility of a previous mid-Cretaceous mineralizing episode at that locality. Although Pb isotopic ratios from sulfides from Fortymile prospects also overlap those of feldspars from 94 to 78 Ma intrusive rocks from east-central Alaska outside the study area, the absence of feldspar Pb isotope data from newly dated ca. 96–94 Ma felsic porphyries associated with the Kechumstuk fault (Fig. 4) precludes our evaluation of this as a possible phase of intrusion-related sulfide mineralization. The paucity of lithologically similar porphyries noted during mapping of the Kechumstuk fault area suggest that possible products of this magmatic episode are volumetrically small and thus unlikely to be the causative intrusions for the epigenetic mineralization.

Intense alteration in the 108.8 ± 1.7 Ma Corner granite sample and ca. 96–94 Ma porphyries and deformed quartz in the Corner sample (Supplemental File 1 [see footnote 1]) suggest the presence of hydrothermal fluids and movement along splays of the Kechumstuk sinistral-normal fault zone and in faults west of the Mount Veta ridge (Fig. 4), either synchronous with intrusion of one or both of the mid-Cretaceous phases of felsic magmatism or during subsequent intrusion of the 68–65 Ma rhyolite dikes that parallel the faults. We propose that the best indication for the timing of hydrothermal fluid flow and associated mineralization for the western Fortymile base and precious metal prospects is the proximity of the prospects to 68–66 Ma intrusions and dikes (Fig. 4). The Oscar skarn prospect is located within ∼300 m of both the Early Jurassic Mount Veta intrusion and the 65.8 ± 1.5 Ma intrusion north of Mount Veta (Fig. 4). Further south, narrow, undated felsic dikes mapped adjacent to northeast-trending faults 3 km north of and along strike from the Eva prospect (Day et al., 2014) appear compositionally similar to the ca. 68 Ma dike along the Kechumstuk fault to the east (Fig. 4), suggesting a likely Late Cretaceous age for the intrusion-related mineralization. A 3-km-long 68.1 ± 0.8 Ma Late Cretaceous felsic dike occurs within a splay of the Kechumstuk fault zone ∼100 m west of the Mitchell prospect (Fig. 4) and leucocratic dikes are present at the prospect (Full Metal Minerals, USA, Inc., 2009, in-house report).

Dip-slip movement along the faults at the LWM played an important role in juxtaposing unreactive metavolcanic footwall rocks against reactive carbonate hanging-wall rocks and channeling metalliferous hydrothermal fluids to their sites of carbonate replacement (Siron et al., 2010). The left-lateral jog in the Kechumstuk fault that formed the pull-apart releasing bend near the LWM prospect also facilitated mineralization (Siron et al., 2010). The location of the West LWM prospect within strands of the Kechumstuk fault system suggests a similar role for fluid flow along the faults; we consider it likely that mineralization there was coeval with that at the LWM. At the Eva Creek prospect, calcite-filled breccia zones as much as 17.7 m thick in drill core and northeast-trending metal anomalies in soils (Dashevsky et al., 1986) are parallel to local faults, suggesting a spatial, and likely genetic, association between deformation and mineralization. A similar association of Late Cretaceous epithermal prospects with northeast-trending faults has been shown in metallogenic and geophysical studies spanning southwestern Yukon and easternmost Alaska (Allan et al., 2013; Sanchez et al., 2013).

At the LWM carbonate replacement prospect, Siron et al. (2010) noted that altered feldspar porphyry dikes are closely associated with, and often in contact with, massive sulfides, and postulated that the dikes may have acted as aquitards, focusing metalliferous hydrothermal fluids along nearly vertical structures. Our U-Pb zircon date of 187.7 ± 2.3 Ma for one of these dikes (6, Table 2; Fig. 5), together with the similar, albeit less robust (28% of 39Ar released), 40Ar/39Ar incremental heating age of 187.5 ± 2.0 Ma for secondary sericitic muscovite from an altered porphyry dike in LWM drill core (Supplemental File 4 [see footnote 4]), suggests that Early Jurassic dikes may have played a role in establishing structural pathways that helped focus the Late Cretaceous hydrothermal fluids. The minor Ar gas loss at ca. 65 Ma in the sample of secondary sericitic muscovite is consistent with the Late Cretaceous age of mineralization permitted by Pb isotopic data and inferred from the spatial association of 68–66 Ma intrusions with some of the prospects noted here. We suggest that this gas loss likely records Late Cretaceous mineralization by fluids at a temperature below the closure temperature of the Ar system and sericitic muscovite.

The Late Cretaceous timing of carbonate replacement and skarn mineralization in the Mount Veta area overlaps with the time of porphyry-related mineralization to the south and east. Newberry et al. (1998a) proposed that a northwest-trending belt of ca. 70 Ma Cu-Mo ± Au porphyry prospects and deposits of the Carmacks belt occurs in west-central Yukon and extends northwestward into the Tanacross quadrangle. At the Mosquito porphyry Cu-Mo-Au prospect at the northwestern end of this belt, south of our study area (prospect C, Fig. 2), vein K-feldspar yielded a 40Ar/39Ar age of 70.0 ± 0.3 Ma interpreted as the age of mineralization (Newberry et al., 1998a). Newberry et al. (1998a) pointed out that this age was comparable to that in the Mount Fairplay alkalic complex that contains minor gold-bearing veins, and proposed that mineralization at Mount Fairplay was part of the Carmacks belt. Our zircon U-Pb ages of ca. 67 Ma confirm a Late Cretaceous, albeit slightly younger, age for Mount Fairplay magmatism. Northeast of Mount Fairplay, granodiorite that crops out in the same general vicinity as the Pika Canyon porphyry Cu prospect (prospect D, Fig. 2) yielded a zircon U-Pb age of ca. 70 Ma (Mortensen, 2000, unpub. data), suggesting that this prospect is also part of the Late Cretaceous Carmacks porphyry belt. Host rocks to Cu-Mo porphyry-style mineralization at the Bluff and adjacent Taurus prospects in the eastern Tanacross quadrangle (prospect E, Fig. 2) and the giant Casino Cu-Mo-Au porphyry deposit in Yukon (south of area shown in Fig. 2) yielded slightly older zircon U-Pb ages of 74–72 Ma (Allan et al., 2013). Both in easternmost Alaska and southwestern Yukon, these prospects are proposed to be genetically associated with northeast-trending faults (Fig. 2) (Sanchez et al., 2013; Allan et al., 2013), consistent with the results of our study indicating a Late Cretaceous age for epithermal mineralization and its association with sinistral and normal movement along the Kechumstuk fault.

Mid-Cretaceous SHRIMP ages for igneous rocks associated with skarn and porphyry Cu-Mo-W-Ag prospects within the Mount Harper batholith in the western part of our study area (Fig. 2) indicate that mineralization accompanied ca. 103 Ma magmatism, in accordance with previous studies in the area and elsewhere in east-central Alaska (Newberry et al., 1998a; Newberry, 2000). At the Section 21 prospect, the correspondence between our 101.4 ± 1.4 Ma zircon U-Pb age for the aplite dike (Fig. 3) and the 102.7 ± 0.4 Ma 40Ar/39Ar age of muscovite from a mineralized vein (Newberry et al., 1998a) indicates that intrusion of felsic dikes and mineralized quartz veins was nearly synchronous. A comparable age for mineralization at the Peternie Cu-Mo porphyry prospect, ∼70 km to the southeast (prospect A, Fig. 2; Werdon et al., 2004a) is suggested by a 102.8 ± 0.5 Ma 40Ar/39Ar age for secondary K-feldspar in a vein that cuts an intrusion that is spatially associated with the Sixtymile Butte caldera; a ca. 110–108 Ma zircon U-Pb age for welded tuffs from the Sixtymile Butte and adjacent calderas (Mortensen and Dusel-Bacon, 2014) provides a maximum age for mineralization at the Peternie prospect.

Mid-Cretaceous hydrothermal activity in the western Fortymile district also overlaps a 104 Ma Re-Os molybdenite age (Selby et al., 2002) that best determines the timing for orogenic and/or intrusion-related Au mineralization within 109–103 Ma, reduced, ilmenite-series igneous rocks at the Pogo mine (Fig. 2) west of our study area (Rhys et al., 2003; Dilworth et al., 2007). In southern Yukon, south of the area shown in Figure 2, metallogenic episodes, including Au-bearing breccia complexes, skarns, and polymetallic veins, are associated with 115–98 Ma magnetite-series arc magmas in the Dawson Range (Allan et al., 2013, and references therein).

A younger, ca. 96–92 Ma, Cretaceous phase of epigenetic mineralization is documented in the intrusion-related prospects of the Mount Harper area and elsewhere in east-central Alaska. The 96.2 ± 1.3 Ma SHRIMP U-Pb age of the porphyry dike at the Section 21 prospect (Fig. 3) corresponds with the timing of mineralization at the nearby Lucky 13 W-rich skarn on Larsen Ridge (prospect A, Fig. 3) proposed by Newberry et al. (1998a) on the basis of a 94.2 ± 0.3 Ma biotite 40Ar/39Ar age of an associated granite. At the Fort Knox gold deposit near Fairbanks (Fig. 2), mineralization consists of gold- and sulfide-bearing quartz veins and associated alteration within a composite stock dated as 92.5 ± 0.2 Ma and 92.4 ± 1.2 Ma by zircon U-Pb and molybdenite Re-Os, respectively (Selby et al., 2002, and references therein), confirming an intrusion-related origin for gold-bearing fluids (McCoy et al., 1997). Skarn and associated Pb-Zn-Ag carbonate replacement mineralization of the Lead Creek and Champion Creek skarn prospects in the eastern Fortymile mining district (prospect B, Fig. 2) also are attributed to this ca. 96–92 Ma magmatic episode on the basis of a TIMS zircon U-Pb age of 96.2 ± 1.0 Ma for felsic porphyry associated with mineralized intervals in Lead Creek drill core and galena Pb isotopes from the two prospects that are within the range of feldspars from mid- and Late Cretaceous intrusions (Dusel-Bacon et al., 2003).

Tectonic Framework of Magmatism

Northeast-Trending Faulting

Wilson et al. (1985) proposed the existence of northeast-trending lineaments that divided the bedrock of the Yukon-Tanana Upland into different domains, based on the regional distribution of K-Ar ages. Subsequent geologic and aeromagnetic data have confirmed and expanded on the significance of the lineaments and identified them as steeply dipping faults with sinistral and oblique-extensional dip-slip displacement (e.g., Dusel-Bacon and Murphy, 2001; O’Neill et al., 2010; Sanchez et al., 2013; Allan et al., 2013). Movement along the northeast-trending faults in the Yukon-Tanana Upland (Fig. 2) has been attributed to the clockwise rotation of blocks resulting from dextral shear along the Tintina and Denali fault systems (e.g., Page et al., 1995; O’Neill et al., 2010). Most movement along these faults is considered to have taken place in early Tertiary time (e.g., Gabrielse et al., 2006), but in Alaska both faults systems show evidence for displacement continuing into the Holocene (Stout et al., 1973; Plafker and Berg, 1994) and, in the Denali fault, to the present (Eberhart-Phillips et al., 2003). O’Neill et al. (2010) postulated that normal and sinistral shear in the northeast-trending Black Mountain tectonic zone in the southwestern Big Delta quadrangle (Fig. 2) began in mid-Cretaceous time, based on the spatial association of ca. 110–95 Ma granitoids and related felsic dikes, plugs, and quartz vein systems with the tectonic zone, with recurrent movement taking place in early Tertiary and Quaternary time.

We propose that mid- to Late Cretaceous northwest-trending dextral faults existed outboard of the domain of northeast-trending oblique-sinistral faults in east-central Alaska and that, like the subsequently developed Denali and Tintina fault systems, caused rotation and wrenching of the inboard crust along the Kechumstuk and other northeast-trending normal-sinistral faults. Mid-Cretaceous dextral, orogen-parallel displacement has been recognized along northwest-trending faults in the northern Cordillera in Canada (Gabrielse et al., 2006; Nelson et al., 2013). The age of these faults in Canada is constrained by ca. 115–100 Ma ages of synkinematic and late kinematic granitic plutons bordering the faults and ca. 110–95 Ma K-Ar dates on muscovite generated during fault movement (Gabrielse et al., 2006).

Normal faults along the margins of the 70 Ma Middle Fork caldera, resulting in preservation of caldera fill (Fig. 3), may have developed during caldera formation or followed it by several million years. A broad temporal association between caldera formation, normal faults, and a Late Cretaceous phase of the northwest-side-up normal dip-slip and sinistral strike-slip (oblique transtensional) displacement along the Kechumstuk fault zone is suggested by U-Pb zircon ages of 68.1 ± 0.8 and 67.7 ± 0.7 Ma for the felsic dike and granitic stock within strands of the Kechumstuk fault. The fault zone constitutes the western margin of the upthrown subvolcanic intrusion, relative to the downdropped 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).

Several lines of evidence indicate recurrent movement along the Kechumstuk fault zone. Graphitic quartzite fault breccia from drill holes along the Kechumstuk fault zone at the LWM prospect consists of subrounded to angular fragments of unaltered marble, dolomitized marble, altered and strongly pyritized dacite porphyry dike (likely coeval with our 187.7 ± 2.3 Ma sample), and rare Zn- and Pb-sulfide mineral clasts in a matrix of graphite with minor disseminated pyrite. This texture indicates that the Kechumstuk faults underwent movement after dacite porphyry dike emplacement and sulfide mineralization (Siron et al., 2010). Recurrent fault movement is also evident in the narrow zones of brittle deformation in northeast-trending faults on either side of the Mount Veta ridge (Fig. 4; Day et al., 2014) that contain evidence of superimposed offset of preexisting brittle deformation, rebrecciated silicic and iron-oxide alteration, and brecciation of younger granitic and rhyolitic dikes and small intrusions.

We speculate that the parallelism and spatial proximity of the Kechumstuk fault zone to the steep, southeast-dipping Early Jurassic Mount Veta intrusion and associated dikes revealed in LWM drill core may indicate that Early Jurassic structures associated with synkinematic intrusion and inferred northwest-vergent contractional deformation based on kinematic studies elsewhere in the YTT (Hansen and Dusel-Bacon, 1998) may have been reactivated by Cretaceous and younger sinistral and normal faulting. The Black Mountain tectonic zone in the Big Delta quadrangle is another example of structural reactivation following mid-Cretaceous faulting in which the tectonic zone served as a tectonic conduit for younger mafic dikes and a Paleocene rhyolite flow-dome complex, with deformation continuing into the Quaternary (Day et al., 2007; O’Neill et al., 2010).

An even older history for the zone of tectonic weakness associated with the northeast-trending Kechumstuk fault zone and associated faults west of the Mount Veta ridge is suggested by the occurrence of the metaharzburgite lens (described above) located between northeast-trending faults and near the western boundary of the YTT and parautochthonous YTa (Fig. 3). In Dusel-Bacon et al. (2013), it was proposed that the metaharzburgite lens in the Mount Veta area, like those in other parts of the Yukon-Tanana Upland, are pieces of the mantle that originated from beneath the Seventymile ocean basin and/or from subcontinental mantle lithosphere of the allochthonous YTT or the western margin of Laurentia and were tectonically emplaced within crustal rocks during closure of the Seventymile ocean basin in late Paleozoic to early Mesozoic time. They speculated that, given the complex Paleozoic and Mesozoic structural and thermal evolution of the allochthonous YTT, outlined here, exposure of the metaharzburgite lens between strands of northeast-trending faults with predominantly Late Cretaceous to early Tertiary sinistral and/or normal movement suggests that the northeast-trending faults may have reused the Paleozoic or early Mesozoic deep crustal structure formed during internal deformation and tectonic transport of the oceanic and pericratonic terranes.

Permian–Cretaceous Tectonic Evolution

Progressive closure of the Slide Mountain–Seventymile ocean occurred along its western margin in middle to late Permian time as a result of a change in plate motion that led to southwest-dipping right-oblique subduction beneath the allochthonous YTT (e.g., Dusel-Bacon et al., 2006; Nelson et al., 2006; Beranek and Mortensen, 2011) (Fig. 13A). U-Pb geochronologic studies in parts of the YTT in Yukon identified a major fabric-forming metamorphic episode that accompanied late Permian crustal thickening (Berman et al., 2007; Beranek and Mortensen, 2011). The 215.0 ± 3.5 Ma SHRIMP U-Pb age for the pluton of Kechumstuk Mountain that is discordant to the regional penetrative fabric establishes a minimum age for the oldest metamorphism recorded in the allochthonous YTT in east-central Alaska.

Following closure of the Slide Mountain–Seventymile ocean in the late Permian, subduction stepped outboard, leading to Triassic magmatism above a newly established east-dipping subduction zone beneath the now reconfigured western margin of North America from Alaska and Yukon to California (Dickinson, 2004; Nelson and Colpron, 2007; Beranek and Mortensen, 2011; Nelson et al., 2013). In east-central Alaska and the northern Canadian Cordillera, Late Triassic arc plutons are restricted to the allochthonous YTT (Fig. 2), suggesting that the parautochthonous YTa was east or north of the zone of arc magmatism (Fig. 13B). Feldspars from Late Triassic rocks have the lowest Pb isotopic ratios of any of the analyzed Mesozoic igneous rocks (Fig. 12) indicating the largest ratio of mantle to crustal components in their source areas. The mantle and upper crustal components of Pb isotopes for feldspars from Late Triassic intrusions in the Fortymile district likely originated from Pb derived from subduction-related mantle and from upper crustal material that formed the basement of both the parautochthonous YTa and the rifted allochthonous YTT.

Early Jurassic magmatism in east-central Alaska accompanied a subsequent episode of east-dipping subduction under the reconfigured continental margin (Fig. 13C) (Beranek and Mortensen, 2011). In the southeastern Eagle quadrangle, weakly foliated Early Jurassic plutons yield U-Pb zircon and titanite ages between ca. 197 and 186 Ma (Table 5). Magmatic epidote is present in three ca. 197 Ma intrusions and one 188 Ma intrusion (Table 5), indicating emplacement of the host plutons at mesozonal crustal depths of >15 km (Werdon et al., 2001; Day et al., 2002; Dusel-Bacon et al., 2009). Magmatic epidote also occurs in the synkinematic and synmetamorphic Early Jurassic (186.0 ± 2.8 Ma) foliated Aishihik batholith that intrudes rocks equivalent to the YTT in southwest Yukon (Johnston and Erdmer, 1995).

Kinematic, metamorphic, and geochronologic studies from the southeastern Eagle quadrangle (Hansen and Dusel-Bacon, 1998; Dusel-Bacon et al., 1995, 2002, 2009) indicate that intrusion of the foliated Early Jurassic granitoids was synkinematic to late kinematic with northwest-vergent (orogen-parallel) contraction that emplaced the Fortymile River assemblage onto the Nasina assemblage 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 rocks in both the parautochthonous Yukon Tanana assemblage and the allochthonous YTT (Fig. 13C). In situ SHRIMP monazite geochronology and pressure-temperature data from rocks of the allochthonous YTT in western Yukon indicate an equivalent Early Jurassic (ca. 195–187 Ma) metamorphic episode that is interpreted to reflect the change from regional contact metamorphism during arc plutonism to internal duplication of the YTT during its collision with the North American craton (Berman et al., 2007). We propose that development of the southeast-dipping thrust fault east of the Kechumstuk fault zone and intrusion of the tabular, synkinematic Early Jurassic Mount Veta intrusion and the southeast-dipping dikes identified in LWM drill core accompanied the northwest-vergent contraction. The 183.4 ± 3.6 Ma age for the intensely altered, but nonfoliated, hornblende-biotite granite porphyry that crops out as a thin sill in the hanging wall of the thrust fault that places Nasina assemblage rocks over Fortymile River assemblage rocks may provide an upper age limit for the thrusting and attendant shortening in the western Fortymile area. The Early Jurassic dikes at the LWM prospect and possibly the contemporaneous syntectonic Mount Veta intrusion may have played a role in subsequent epigenetic mineralization by providing structural pathways that helped focus the Late Cretaceous metalliferous hydrothermal fluids along the steep northeast-trending faults that likely reused the zones of weakness established during Early Jurassic contraction.

In the southeastern Eagle quadrangle, 40Ar/39Ar plateau ages of hornblende and biotite from Early Jurassic granitoids that intrude the Fortymile River assemblage (Table 5) and hornblende, muscovite, and biotite from the Fortymile River assemblage and the structurally underlying Nasina assemblage indicate rapid cooling from ∼450 °C to ∼350 °C at ca. 188–186 Ma (Hansen et al., 1991; Dusel-Bacon et al., 2002, and references therein). The tectonic origin of the rapid cooling and exhumation is unclear and possible explanations include (1) gravitational collapse of overthickening crust, (2) uplift via thrusting and erosion (Hansen and Dusel-Bacon, 1998; Dusel-Bacon et al. 1995, 2002; Day et al., 2002), and (3) upper plate extension and transtensional strike-slip (sinistral?) faults as a result of slab rollback (Berman et al., 2007).

Subsequent oblique northeast-dipping subduction occurred beneath the combined allochthonous YTT and parautochthonous YTa and resulted in the accretion of the Wrangellia composite terrane to the western margin of North America (e.g., Plafker and Berg, 1994) (Fig. 13D). The Wrangellia composite terrane (also known as the Insular terrane in Canada) is a subcontinental-scale crustal fragment consisting of the joined Wrangellia, Peninsular, and Alexander terranes that contains Devonian through mid-Cretaceous arcs (Nokleberg et al., 1994). The Wrangellia composite terrane extends from southern Alaska to British Columbia, was located several thousand kilometers south of its present position during the Late Triassic, and was translated northward and accreted to the western margin of North America during Middle Jurassic to Late Cretaceous time. The timing and location for the initial accretion is uncertain owing to differing interpretations of paleomagnetic and geologic data and the history of postaccretion translation (e.g., Nokleberg et al., 1985; Trop et al., 2002).

In east-central Alaska, 40Ar/39Ar metamorphic cooling ages of ca. 135–110 Ma are widespread and interpreted to record mid-Cretaceous exhumation of the lower plate parautochthonous YTa as a result of slab rollback of the subducting plate (Pavlis, 1989; Hansen, 1990; Hansen et al., 1991; Pavlis et al., 1993; Hansen and Dusel-Bacon, 1998) (Fig. 13D). Exhumation occurred during southeast-vergent (orogen parallel) extensional ductile deformation and metamorphism documented throughout much of east-central Alaska (Hansen and Dusel-Bacon, 1998). An apparent top-to-the-south-southeast fabric was observed in tonalitic mylonite gneiss in the Chicken metamorphic complex east of the Kechumstuk fault (Dusel-Bacon et al., 2013) and in a few other locations in the nearby Molly Creek area just west of the Mount Veta ridge (Hansen and Dusel-Bacon, 1998). Top-to-the-southeast-vergent deformational fabrics overprint the earlier top-to-the-northwest fabrics in our study area, and like elsewhere in the region, the younger fabrics are interpreted to be generally structurally shallower than the older fabrics and to have formed during regional extension (Hansen and Dusel-Bacon, 1998). An analogous domain of late-Early Cretaceous ca. 118–112 Ma exhumation of originally structurally lower rocks has been identified, based on monazite SHRIMP U-Th-Pb ages and thermobarometry in amphibolite facies metasedimentary schists, in the Australia Mountain area in west-central Yukon, and is interpreted to represent a tectonic window into a metamorphic core of the YTT, or perhaps the lower plate parautochthonous YTa (Staples et al., 2013).

Widespread intrusion of mid-Cretaceous calc-alkaline arc granitoids, including the ca. 112–94 Ma dated bodies in our study, occurred throughout the Yukon-Tanana Upland following mid-Cretaceous regional extension (Fig. 13D) (Wilson et al., 1985; Newberry et al., 1998b). Whole-rock trace element data and Pb isotope compositions for feldspars from the mid-Cretaceous intrusions from our study indicate a significant crustal component in the magmas, likely generated during crustal thickening prior to extension. This conclusion is in accord with Hart et al. (2004) who, on the basis of aeromagnetic signatures, magnetic susceptibilities, and whole-rock ferric:ferrous ratios, determined that 109–102 Ma intrusions in the Yukon-Tanana Upland form an ilmenite-series belt that developed from melting of continental crust in response to crustal thickening associated with terrane collision. Although some of vein and skarn mineralization in the Mount Harper batholith and elsewhere in east-central Alaska occurred within this magmatic episode, the role of these granitoids in generating the economically significant gold mineralization at the Pogo deposit is equivocal (Hart et al., 2004, and references therein). Mid-Cretaceous plutonism and development of the ca. 108 Ma rhyolite calderas in the Tanacross quadrangle (Fig. 2) may have been facilitated by decreasing pressure during extension-related exhumation (Dusel-Bacon et al., 1995) (Fig. 13D).

After formation of the Kula plate at ca. 85 Ma (Plafker and Berg, 1994), landward-dipping subduction continued along a new, outer subduction zone (Fig. 13E). Northward motion of the Kula–Farallon–North American plate triple junction resulted in dextral-oblique compression of the continental margin and northwestward movement of the now combined allochthonous YTT and parautochthonous YTa along strike-slip faults that likely evolved or integrated into the younger right-lateral Tintina and Denali fault systems and their extensions (Plafker and Berg, 1994). In southern Yukon, ca. 79–72 Ma small intrusions are spatially associated with, and locally controlled by, the northwest-trending, dextral Big Creek fault that cuts the YTT between and parallel to the Denali and Tintina fault systems (Bennett et al., 2010). In east-central Alaska, subduction is reflected in ca. 75–65 Ma granitoids in the northwestern Yukon-Tanana Upland (Wilson et al., 1985) and the ca. 70–66 Ma Middle Fork caldera, plutons, and rhyolite dikes along the Kechumstuk fault zone, and the Mount Fairplay intrusion.

Crustal enrichment is indicated by the 206Pb/204Pb isotopic ratios of feldspars from the ca. 70–66 Ma igneous rocks in the western Fortymile area, but their 207Pb/204Pb and 208Pb/204Pb ratios imply that the parental source material of this age group contains a smaller upper crustal component and/or a larger mantle component than was present in the source of the mid-Cretaceous magmas. The tectonic setting for this mantle input is unknown, but we propose that after the outer subduction zone of the Kula-Farallon plate was established, the trapped and now inactive (fossil) inner subduction zone was dominated by the effects of lithospheric sinking that culminated in slab breakoff and led to asthenospheric upwelling (Fig. 13E). Such a scenario could explain the Late Cretaceous juvenile Pb isotopic signatures and peraluminous and metaluminous compositions in our study, the within-plate and syncollisional tectonics settings indicated for multiple areas of interior Alaska (Newberry, 2000), and the mantle signature of some volcanic rocks in the Late Cretaceous Carmacks Group in Yukon (Johnston et al., 1996). The rapid cooling indicated by the 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 calc-alkaline intermediate and felsic volcanic rocks of the Carmacks Group in southwestern Yukon are lithospheric delamination (Mortensen and Hart, 2010) and plume-related magmatism (Johnston et al., 1996).

In east-central Alaska, this dextral-oblique compression of the continental margin and northwestward movement of the crystalline rocks between the right-lateral Tintina and Denali fault systems and their precursor faults was key to the far-field development of the northeast-trending sinistral-normal faults that were associated with Late Cretaceous magmatism and epigenetic mineralization in the Fortymile district and adjacent areas of Yukon.

CONCLUSIONS

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 Triassic (216–208 Ma), Early Jurassic (199–181 Ma), mid-Cretaceous (112–94 Ma), and Late Cretaceous (70–66 Ma) pulses of arc magmatism. The tabular Early Jurassic Mount Veta intrusion and coeval dikes at the LWM prospect are synkinematic bodies that likely accompanied margin-parallel northwest-directed crustal shortening during northwest-dipping subduction.

Epigenetic Pb-Zn-Ag ± Cu prospects in the western Fortymile district are spatially associated with splays of the northeast-trending Kechumstuk sinistral-normal fault zone and related faults and with ca. 68–66 Ma felsic intrusions and dikes. Similarity between Pb isotope compositions of feldspars from Late Cretaceous intrusions and sulfides from the prospects and proximity of prospects to the ca. 68–66 Ma bodies suggest a Late Cretaceous age for most of the mineralization. Mineralizing hydrothermal fluids clearly were channeled along the northeast-trending faults. Intrusion of ca. 68–66 Ma felsic dikes and a stock, as well as older ca. 95 Ma hypabyssal bodies, within the northeast-trending Kechumstuk fault zone was apparently synchronous with recurring sinistral and normal fault displacement. We interpret fault displacements to be a far-field effect of dextral translation along mid- to Late Cretaceous plate-scale boundaries and faults that were roughly parallel to the subsequently developed, largely Cenozoic, Denali and Tintina fault systems.

The Pb isotopic compositions of feldspars are more radiogenic with decreasing age, consistent with the magmas being mantle derived but extensively contaminated by upper crustal components with evolving Pb isotopic compositions. Feldspars from Late Cretaceous igneous rocks have a larger juvenile Pb component relative to compositionally similar rocks of mid-Cretaceous age, suggesting input of mantle material during generation of Late Cretaceous magmas. The tectonic origin of this mantle input is uncertain, but may have been related to asthenospheric upwelling resulting from sinking of the inactive inner subduction zone that delivered the previously accreted Wrangellia composite terrane after the outer Farallon subduction zone was established.

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 the Mount Veta area. Doyon, Ltd. granted permission required to conduct our studies on Doyon’s selected or conveyed lands in the western Fortymile study area. Charlie Bacon supplied invaluable help in the field, including sample collection and logistics. John Slack and Mike O’Neill are 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 for us to publish the 40Ar/39Ar data. We appreciate the efforts of Joseph Wooden in ensuring that the Stanford–U.S Geological Survey instrument worked well during our analytical sessions. We 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 feldspar samples at the Pacific Centre for Isotopic and Geochemical Research, University of British 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. Geological Survey Mineral Resources Program.

1Supplemental File 1. Map numbers, sample numbers, location information, and detailed petrographic description of each SHRIMP U-Pb zircon sample employed in the study of the Mesozoic igneous rocks from the Fortymile district, east-central Alaska. Please visit http://dx.doi.org/10.1130/GES01092.S1 or the full-text article on www.gsapubs.org to view Supplemental File 1.
2Supplemental File 2. SHRIMP U-Pb zircon data from the Fortymile district, east-central Alaska. Part A describes the analytical methods used for U-Pb geochronology of zircon. Part B presents representative cathodoluminescence images of zircon, concordia plots, and weighted average plots. Part C presents information used in the interpretation for analytical results for each dated sample, including the morphology and cathodoluminescence zoning characteristics of its zircon. Please visit http://dx.doi.org/10.1130/GES01092.S2 or the full-text article on www.gsapubs.org to view Supplemental File 2.
3Supplemental File 3. SHRIMP U-Th-Pb data for zircon from rocks of the Fortymile district, east-central Alaska. Please visit http://dx.doi.org/10.1130/GES01092.S3 or the full-text article on www.gsapubs.org to view Supplemental File 3.
4Supplemental File 4. 40Ar/39Ar age and compositional data from incremental heating of secondary sericitic muscovite from altered felsic porphyry dike in LWM drill core. Please visit http://dx.doi.org/10.1130/GES01092.S4 or the full-text article on www.gsapubs.org to view Supplemental File 4.
5Supplemental File 5. Location data and crystallization ages for whole-rock geochemistry samples of felsic and mafic igneous rocks from the Fortymile district, east-central Alaska, and the map number for samples analyzed or dated in this study or the citation for previously published geochemical or geochronological data. Please visit http://dx.doi.org/10.1130/GES01092.S5 or the full-text article on www.gsapubs.org to view Supplemental File 5.