Geologic Studies in Alaska by the U.S. Geological Survey, 1992

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L wiseman thrust 0 10 20 miles I 0 I 10 I I 20 I kilometers I W +nu LT O PC o-la zcnw I- cn Ezs o ~ [iiii EXPLANATION Cambrian to Early Cretaceous sedimentary and volcanic rocks Late Devonian to Early Cretaceous sedimentary rocks Nutirwik Creek unit--Devonian o w felsic metavolcanic rocks zz 9 Phyllite and schist xu ' 2 S+ -l e Schist Ordovician to Middle Devonian metacarbonate rocks Devonian metagranitic rocks SLATE CREEK TERRANE ANGAYUCHAM TERRANE Shallow to moderate dipping fault--Sawteeth on upper plate Contact 88TM13! U-Pb sample locality and number Figure 2. Simplified geologic map of part of eastern Brooks Range, Alaska (modified from Dillon and others, 1986; BrosgC and Reiser, 1964; and unpublished mapping of the TACT program). Many units have been combined within each terrane.
U-Pb AGES OF ZIRCON, MONAZITE, AND SPHENE FROM BROOKS RANGE, ALASKA Table 1. Description of rocks and minerals from dated samples Sample Name and number minerals dated Brief descriotion 90TM408 Geroe Creek orthogneiss zircon sphene 90TM498 Horace Mountain plutons zircon sphene 90ANKO12 Baby Creek orthogneiss zircon monazite 88TM 137 Middle Fork Koyukuk River orthogneiss body zircon COLDFOOTTERRANE Coarse-grained, quartz-K-feldspar-plagioclase-hornblende-biotite; strongly foliated, with quartz-sericite shear bands; fine-grained mosaic quartz; strained large quartz grains. Euhedral, medium-brown, length-to-width ratio (llw) 3-5, mottled faces. Pale yellow, adamantine, euhedral fragments. Coarse-grained, quartz-K-feldspar-plagioclase-hornblende-biotite; some amphiboles contain relict cores of clinopyroxene; all feldspars are highly sericitized; strongly foliated, mortar structure, mosaic texture; strained quartz. Euhedral, light brown, Vw 2-4, adamantine. Pale yellow, adamantine, euhedral fragments. Medium-grained, quartz-microcline-plagioclase-biotite-muscovite; phenocrysts of microcline; strongly foliated, with shear bands of quartz-sericite-biotite; finegrained mosaic quartz. Medium-brown, llw 2-6, mottled faces. Subhedral, pale yellow-orange, opaque. Coarse-grained, quartz-muscovite-microcline-plagioclase-calcite-gnet; strongly foliated coarse-grained knots of clinozoisite-quartz-muscovite. Subhedral. dark brown. Vw 1-5, deformed. HAMMOND TERRANE 90TM409 dacite porphyry (Nutirwik Creek unit) zircon Fine-grained, buff, strongly foliated, phenocrysts of quartz and plagioclase, now rotated and polygonized; stretched fragments; altered to sericite and carbonate. Euhedral, light pink, Vw 2-4, adamantine. 90TM497 plagioclase porphyry Fine-grained, light green; phenocrysts of plagioclase and hornblende; slightly foliated (Nutirwik Creek unit) to unfoliated; hornblende replaced by epidoteand chlorite; groundmass is fine- zircon and lithologically correlative. For the purpose of this pa- per, we geographically restrict the Baby Creek batholith (hereafter referred to as the Baby Creek orthogneiss body) to the two southern orthogneiss bodies at Baby Creek and to the nearby Twin Lakes body. The Baby Creek orthogneiss body appears to be a thick but tabular, fault- bounded body that is locally intrusive into adjacent meta- morphosed mafic volcanic rocks. The Horace Mountain plutons, Geroe Creek orthogneiss body, and Baby Creek orthogneiss body display strong foliation, local mylonite zones, and gneissic textures along their outer contacts. Al- though foliated, relict igneous features such as xenoliths and megacrysts of potassium feldspar are locally retaiiled in the cores of most of the bodies. The small, poorly exposed orthogneiss body located on the Middle Fork of the Koyukuk River (hereafter referred to as the Middle Fork Koyukuk River orthogneiss body for convenience) south of Wiseman is medium- to coarse-grained, gneissic, muscovite-biotite quartz monzo- nite. This body locally contains garnet and megacrysts of microcline (Dillon and Reifenstuhl, 1990). Its contact with overlying calcschist is unexposed, but the absence of dikes or stocks suggests that the contact is a low-angle fault. grained mosaic of quartz and feldspar, kith abundant needles of pale amphibole. Euhedral, light pink, Vw 4-8, adamantine. Metagranitic rocks of the central Brooks Range are as- sociated with porphyry, polymetallic vein, and skarn rnin- era1 deposits. Lode occurrences include polymetallic quartz veins with base-metal sulfides, tin skarns with disseminated cassiterite and base-metal sulfides, Cu-Pb-Zn skarns with disseminated iron and base-metal sulfides, and porphyry copper and molybdenum (Newberry and others, 1986; Nokleberg and others, 1988). These granitic magmatism-related deposits are part of a 900-km-long belt of mineral deposits in the Coldfoot, Hammond, and North Slope terranes. HAMMOND TERRANE The Hammond terrane in the TACT conidor consists of a variety of lenticular, mostly fine-grained metaclastic, metavolcanic, and carbonate units that are completely to incompletely recrystallized to greenschist assemblages (fig. 2). In the southern part of the study area, rocks of the Hammond terrane display ductile folds, but to the north the rocks are deformed by imbricate thrusts. Primary textures are locally present and increase in abundance northward.
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Geologic Studies in Alaska by the U
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CONTENTS Introduction Cynthia Dusel
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CONTENTS CONTRIBUTORS TO THIS BULLE
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GEOLOGIC STUDIES IN ALASKA BY THE U
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LATE HOLOCENE LONGITUDINAL AND PARA
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Page 17 and 18: LATE HOLOCENE LONGITUDINAL AND PARA
Page 19 and 20: DEEP-WATER LITHOFACIES AND CONODONT
Page 25 and 26: Table 1. Locality register for key
Page 27 and 28: Table 1. Locality register for key
Page 37 and 38: LITHOFACIES AND CONODONTS OF CARBON
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Page 51 and 52: Table 1. Conodont faunules and lith
Page 55 and 56: DEPOSITIONAL SEQUENCES IN ATIGUN SY
Page 65 and 66: U-Pb AGES OF ZIRCON, MONAZITE, AND
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Page 77 and 78: APPEAL FOR NONPROLIFERATION OF ESCA
Page 85 and 86: FAVORABLE AREAS FOR METALLIC MINERA
Page 97 and 98: GOLD AND CINNABAR IN HEAVY-MINERAL
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EARLY CENOZOIC DEPOSITIONAL SYSTEMS
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RESERVOIR FRAMEWORK ARCHITECTURE, C
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GEOCHEMISTRY OF OPHIOLITIC ROCKS FR
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Table 1. Isotopic ages of intrusive
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Table 1. Isotopic ages of intrusive
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GEOCHEMICAL EVALUATION OF STREAM-SE
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Table 3. Summary of geochemical sig
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GEOCHEMICAL CHARACTER OF UPPER PALE
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RECONNAISSANCE GEOCHEMISTRY OF BASA
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OSTRACODE ASSEMBLAGES FROM MODERN B
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RUBIDIUM-STRONTIUM ISOTOPIC SYSTEMA
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RUBIDIUM-STRONTIUM ISOTOPIC SYSTEMA
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US. GEOLOGICAL SURVEY REPORTS ON AL
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U.S. GEOLOGICAL SURVEY REPORTS ON A
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U.S. GEOLOGICAL SURVEY REPOR TS ON
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REPORTS ABOUT ALASKA IN NON-USGS PU
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REPORTS ABOUT ALASKA IN NON-USGS PU
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Geologic Studies in Alaska by the U.S. Geological Survey, 1992

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