Research Paper GEOSPHERE Mesozoic magmatism and timing of epigenetic Pb-Zn-Ag mineralization in the western Fortymile mining district, east-central Alaska: Zircon GEOSPHERE; v. 11, no. 3 U-Pb geochronology, whole-rock geochemistry, and Pb isotopes DOI:10.1130/GES01092.1 Cynthia Dusel-Bacon1, John N. Aleinikoff2, Warren C. Day3, and James K. Mortensen4 1U.S. Geological Survey, 346 Middlefield Road, MS 901, Menlo Park, California 94025, USA 13 figures; 5 tables; 5 supplemental files 2U.S. Geological Survey, Denver Federal Center, MS 963, Denver, Colorado 80225, USA 3U.S. Geological Survey, Denver Federal Center, MS 911, Denver, Colorado 80225, USA 4University of British Columbia, 2020-2207 Main Mall, Vancouver, British Columbia, V6T 1Z4, Canada CORRESPONDENCE: [email protected] CITATION: Dusel-Bacon, C., Aleinikoff, J.N., Day, ABSTRACT on the northeast-trending faults to be a far-field effect of dextral translation W.C., and Mortensen, J.K., 2015, Mesozoic magma- tism and timing of epigenetic Pb-Zn-Ag mineralization along Late Cretaceous plate-scale boundaries and faults that were roughly in the western Fortymile mining district, east-central The Mesozoic magmatic history of the North American margin records parallel to the subsequently developed Denali and Tintina fault systems, which Alaska: Zircon U-Pb geochronology, whole-rock geo- the evolution from a more segmented assemblage of parautochthonous and currently bound the region. chemistry, and Pb isotopes: Geosphere, v. 11, no. 3, p. 786–822, doi:10.1130/GES01092.1. allochthonous terranes to the more cohesive northern Cordilleran orogenic belt. We characterize the setting of magmatism, tectonism, and epigenetic Received 11 June 2014 mineralization in the western Fortymile mining district, east-central Alaska, INTRODUCTION Revision received 25 November 2014 where parautochthonous and allochthonous Paleozoic tectonic assemblages Accepted 28 April 2015 are juxtaposed, using sensitive high-resolution ion microprobe (SHRIMP) U-Pb The Fortymile mining district of east-central Alaska (Cobb, 1973) is located Published online 13 May 2015 zircon geochronology, whole-rock geochemistry, and feldspar Pb isotopes of near the juxtaposition of two Paleozoic tectonic assemblages that developed Mesozoic intrusions and spatially associated mineral prospects. New SHRIMP along the northwestern margin of Laurentia, one parautochthonous and the U-Pb zircon ages and published U-Pb and 40Ar/39Ar ages indicate four episodes other allochthonous (Fig. 1). The middle to late Paleozoic evolution of these of plutonism in the western Fortymile district: Late Triassic (216–208 Ma), Early and other related pericratonic assemblages of the northern Cordillera has been Jurassic (199–181 Ma), mid-Cretaceous (112–94 Ma), and Late Cretaceous well documented (e.g., Dusel-Bacon et al., 2006; Nelson et al., 2006; Piercey et (70–66 Ma). All age groups have calc-alkalic arc compositions that became al., 2006). However, fewer studies have addressed the Mesozoic amalgama- more evolved through time. Pb isotope compositions of feldspars from Late tion to postamalgamation magmatic history of east-central Alaska (Newberry Triassic, Early Jurassic, and Late Cretaceous igneous rocks similarly became et al., 1998a; Hansen and Dusel-Bacon, 1998; Dusel-Bacon et al., 2002, 2009; more radiogenic with time and are consistent with the magmas being man- Allan et al., 2013). Mesozoic magmatism records the interaction of outboard tle derived but extensively contaminated by upper crustal components with allochthonous terranes with the inboard amalgamated continental margin and evolving Pb isotopic compositions. Feldspar Pb isotopes from mid-Cretaceous thus helps constrain plate tectonic models of the northern Cordillera. Mesozoic rocks have isotopic ratios that indicate magma derivation from upper crustal intrusive rocks in the Fortymile mining district are also important because of sources, probably thickened mid-Paleozoic basement. The origin of the mantle their spatial association with undated epigenetic mineral prospects. The dis- component in Late Cretaceous granitoids suggested by Pb isotopic ratios is trict is known for its placer gold (Yeend, 1996), and includes dozens of epigen- uncertain, but we propose that it reflects asthenospheric upwelling following etic base and precious metal prospects (Werdon et al., 2004a; Dusel-Bacon et slab breakoff and sinking of an inactive inner subduction zone that delivered al., 2003, 2009; Siron et al., 2010; Allan et al., 2013), especially in the western the previously accreted Wrangellia composite terrane to the North American part of the district in the southwestern Eagle 1° × 3° quadrangle (Figs. 2 and 3). continental margin, after the outer Farallon subduction zone was established. Our study documents magmatic events, structures, and regional tectonics Epigenetic Pb-Zn-Ag ± Cu prospects in the western Fortymile district are that are important to understanding the metallogeny and mineral potential of this spatially associated with splays of the northeast-trending Kechumstuk sinis- economically important time frame in the Fortymile district, in adjacent Yukon tral-normal fault zone and with ca. 68–66 Ma felsic intrusions and dikes. The (Allan et al., 2013), and throughout much of Alaska (Goldfarb, 1997). This paper similarity between Pb isotope compositions of feldspars from the Late Creta- complements a new geological map of the Mount Veta area (by Day et al., 2014), ceous igneous bodies and sulfides from the epithermal prospects suggests a guided by airborne geophysical surveys (Burns et al., 2008), that provides a struc- For permission to copy, contact Copyright Late Cretaceous age for most of the mineralization. Fluid flow along the faults tural framework for understanding the metallogeny and tectonic evolution of the Permissions, GSA, or [email protected]. undoubtedly played a major role in mineralization. We interpret displacement study area. We present petrologic, whole-rock major, minor, and trace element © 2015 Geological Society of America GEOSPHERE | Volume 11 | Number 3 Dusel-Bacon et al. | Mesozoic magmatism and mineralization, Fortymile district, Alaska Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/3/786/3336037/786.pdf 786 by guest on 27 September 2021 Research Paper Ko 68 tzeb o W N o Soundue Beaufort AK 140 Fortymile Sea mining SD district o W Y.T. 160 Area of eastern B.C. Kaltag fault map 67oN NW T. Y. 12 T 4 Fb o limit W pYTa of Fig. 3 E YT C D ordill W Tintina fault o T Fig.. 2 SEL eran 148 Denali fault YT WYN BASIN Oceanic assemblage Paleozoic - Jurassic Tozitna - Angayucham terranes YT deformation Devonian - Triassic Finlayson Slide Mountain - Seventymile AK Lake terranes Y.T. YT Arc assemblages Wh YT Devonian - Permian Stikine - Boswell assemblage WL Y.T. 60o N Permian 59o Klondike assemblage N B.C. Pennsylvanian - Permian Pacific nana terrane Klinkit assemblage Ta Devonian - Mississippian Ocean kon- Fortymile River, Snowcap and Yu Finlayso n assemblages Continent margin assemblages YT Ta Devonian - Mississippian Alaska Range and pY Yukon-Tanana Upland Neoproterozoic - Devonian North American basinal facies Neoproterozoic - Devonian 56oN North American platformal facies 128 Scale AK Neoproterozoic - Paleozoic 0200 B.C. o Other continent margin W assemblages km Figure 1. Paleozoic tectonic assemblages of the northern Cordillera (modified from Dusel-Bacon et al., 2006; Nelson et al., 2013). Map abbreviations for lithotectonic terranes and assemblages: pYTa—parautochthonous Yukon-Tanana assemblage; YT—allochthonous Yukon-Tanana terrane. Place abbreviations: AK—Alaska; B.C.— British Columbia; D—Dawson; E—Eagle; Fb—Fairbanks; NWT—Northwest Territories; Wh—Whitehorse; WL—Watson Lake; T—Tok; YT—Yukon. Unpatterned areas within the Cordillera are undivided accreted terranes. Red rectangle shows the location of study area. Location of the Fortymile mining district is shown in inset. GEOSPHERE | Volume 11 | Number 3 Dusel-Bacon et al. | Mesozoic magmatism and mineralization, Fortymile district, Alaska Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/3/786/3336037/786.pdf 787 by guest on 27 September 2021 Research Paper ARC AND BASINAL ASSEMBLAGES 148 30′ 147 00′ 141 00′ (ALLOCHTHONOUS YUKON-TANANA TERRANE) 66 00′ 144 00′ Klondike Schist Permian felsic metaigneous rocks; minor metasedimentary and mafic metaigeous rocks Nasina assemblage Qtz-Ser-Chl schist, marble; carbonaceous rocks; Mississippian and Permian felsic metaigneous rocks OCEANIC & HIGH-P ASSEMBLAGES Chicken metamorphic complex Intermediate-mafic metaigneous Seventymile terrane rocks, quartzite, Mississippian(?) marble Peridotite, greenstone; Mississippian to Fortymile River assemblage Bt schist and gneiss, marble, Triassic chert and limestone quartzite; Mississippian metaigneous gneiss (brown) Chatinika assemblage (near Fairbanks) Ladue River unit Qtz-Chl-Ser schist and mafic and felsic Eclogite, marble, schist metaigneous rocks (in part Devonian-Mississippian) Wickersham grit unit iver R Yukon R Figure 2. Generalized geologic map of ive CONTINENTAL MARGIN ASSEMBLAGES Birch Creek r (PARAUTOCHTHONOUS YUKON-TANANA ASSEMBLAGE) east-central Alaska and adjacent part of a A Yukon showing Mesozoic and Cenozoic nik Tintina faul Butte assemblage, Alaska Range equivalents ata Ch Devonian-Mississippian metavolcanic rocks, phyllite, marble granitoids, Paleozoic arc, basinal, and Blackshell and Dan Creek units; Keevy Peak Fm. Carbonaceous continental margin assemblages,