USGS Professional Paper 1697 - Alaska Resources Library

236 Metallogenesis and Tectonics of the Russian Far East, Alaska, and the Canadian Cordillera
to (4), the host rock of the Yukon-Tanna terrane consists of
metamorphosed and penetratively deformed, middle Paleozoic
and older, quartz-metasedimentary, sparse metavolcanic, and
rare middle Paleozoic metagranitoid rocks (Foster and others,
1987). For the deposits in group (5), the granitoid rocks hosting
the felsic-plutonic U deposits intrude a sequence of weakly
deformed, quartz-rich sandstone, grit, shale, and slate, containing
probable Early Cambrian fossils, which form part of the
Wickersham passive continental-margin terrane (Jones and others,
1987). Yukon Territory part of the belt contains porphyry
Cu-Mo deposits in the Carmacks area.
Casino Porphyry Cu-Mo-Au Deposit
The large Casino (Patton Hill) porphyry Cu-Mo-Au
deposit in the Carmacks area, Yukon Territory, is hosted in a
breccia pipe and associated porphyritic dacite and granite of
the Casino Intrusive Complex that has a K-Ar isotopic age of
71.3 Ma and intrudes the Early Jurassic Klotassin batholith.
The deposit occurs in the center of the complex and consists of
a central, conical breccia pipe, fine-grained quartz monzonite,
intrusion breccia, and plagioclase porphyry intrusions (EMR
Canada, 1989; Mining Review, 1992; Sinclair, 1986; Northern
Miner, December 6, 1993; Bower and others, 1995). An innermost
zone of potassic alteration is associated with hypogene
sulfides and contains superimposed phyllic and pyrite alteration
and weakly developed, peripheral propylitic alteration.
Supergene minerals are chalcocite, digenite, and covellite that
occur as replacements of hypogene sulfides. A 70-m-thick,
Au-bearing leached cap overlies a well developed supergene
enrichment blanket. Estimated reserves are (1) for a combined
Au-oxide zone in the leached cap and an copper-oxide supergene
zone, 28 million tonnes grading 0.68 g/t Au, 0.11 percent
Cu, and 0.024 percent Mo, (2) for a sulfide supergene zone, 86
million tonnes grading 0.41 g/t Au, 0.43 percent Cu, and 0.031
percent Mo, and (3) for a hypogene zone, 445 million tonnes
grading 0.27 g/t Au, 0.23 percent Cu, and 0.024 percent Mo
(Godwin, 1976; Bower and others, 1995).
Taurus Porphyry Copper-Molybdenum Deposit
The Taurus porphyry Cu-Mo deposit (E.R. Chipp, written
commun., 1984; Leriche, 1995) occurs in the eastern part of
the Yukon-Tanana Upland in east-central Alaska and consists
of chalcopyrite, molybdenite, and pyrite that occur as disseminations
and in veinlets composed of quartz-orthoclase-sericite,
quartz-magnetite-anhydrite, quartz-sericite-pyrite-clay-fluorite,
and quartz-orthoclase-biotite. The disseminations and
veinlets occur in at least three areas in a zone of hypabyssal
plutons about 13 km long and 1.6 km wide. The plutons
consist of early Tertiary granite porphyry, granodiorite, and
quartz latite porphyry (57 Ma, T.K. Bundtzen, written commun.,
1994) that intrude middle Paleozoic or older quartzmuscovite
schist of the Yukon-Tanana terrane. The core of the
pluton exhibits potassic alteration, whereas sericitic alteration
occurs in the periphery and adjacent wall rocks. The sequence
of alteration, from oldest to youngest, is hydrothermal potas-
sic, propylitic, phyllic, and sericitic. Magnetite-rich cores of
the potassic-altered granite porphyries contain sparse sulfides.
Higher concentrations of Cu and Mo sulfides occur in the
periphery along with phyllic alteration. Tourmaline, fluorite,
and replacement of chalcopyrite by chalcocite occur locally.
Numerous faults and shears occur in the deposit. The deposit
contains a resource of 23 million tonnes grading 0.3 percent
Cu and 0.039 percent Mo (Leriche, 1995).
Road Metal Tourmaline-Topaz-Quartz-Sulfide
Greisen Deposit
The Road Metal deposit displays the classic features of
a tourmaline-topaz-quartz sulfide greisen deposit (Bundtzen,
Schaefer, and Dashevsky, 2003). The deposit occurs adjacent
to the Alaska Highway in East-Central Alaska. 40 Ar/ 39 Ar
isotopic ages of 68 to 70 Ma are obtained from hydrothermal
sericite in veins. The deposit consists of structurally controlled,
sulfide-rich, tourmaline-muscovite greisen, semimassive
sulfide replacement veins, massive white mica alteration
zones, and smokey-quartz-rich, potassic alteration zones
that are hosted in granite and quartz monzonite with a K-Ar
isotopic age of 95 Ma. However, sulfide-rich quartz monzonite
and white mica alteration minerals in greisen yield 40 Ar/ 39 Ar
isotopic ages ranging from 70 to 67 Ma. Sulfide and sulfosalt
minerals in drill core include kobellite, boulangerite, owyheeite,
galena, chalcopyrite, pyrite, and tetrahedrite. The sulfide
greisens strike northeast, dip vertically, range from 40 to 50
meters wide and extend for about 1,400 m along strike. More
than 20 diamond drill holes totaling about 5,300 m of drilling
have been completed and significant mineralization occurs in
about half of the drill holes. As much as 85.8 g/t Au, 4,634 g/t
Ag, 6.0 percent Sb, 2.42 percent Bi, and 3.83 percent Cu occur
in 1.5-meter-long drill intercepts. No resource or reserve estimates
are yet established. A preliminary quartz fluid inclusion
study in mineralized greisen indicates primary fluid inclusions
have homogenization temperatures from 322 to 381º C. and
contain 16.0 to 18.2 weight percent NaCl (Cameron Rombach
and John Mair, University of Western Australia, written
commun., 2001). Secondary inclusions have homogenization
temperatures ranging from 277-430º C and contain 23.1 to
29.9 weight percent NaCl. These results are consistent with
formation in a relatively high temperature saline greisen or a
porphyry Cu-Au system at pressures of 1 to 3 kilobars.
The Road Metal deposit shares some features with other
granitoid-related deposits in the East-Central Alaska metallogenic
belt (younger part) in the Yukon-Tanana Upland,
including high Au and Ag, elevated Bi, major oxide chemistry
of host intrusive rocks, and regional geologic setting. A high
Bi:Au ratio of about 250:1, high Ag:Au ratio of about 40:1,
elevated Sn and W, and intrusive and deposit isotopic ages of
67 to 70 Ma, collectively suggest that the Road Metal deposit
is comparable with deposits hosted in the Carmacks Plutonic
Suite in the western Yukon Territory, with isotopic ages of
75 to 55 Ma, and also with other deposits in the East-Central
Alaska metallogenic belt, including the Casino, Taurus, and