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

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 re-
ferred 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 dis-
seminated 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.